LITHOSTRATIGRAPHY

This section outlines the procedures for documenting the basic sedimentology of the sedimentary deposits recovered in cores drilled during Leg 183, including core description, XRD, color spectrophotometry, and smear-slide preparation. Only general procedures are outlined, except where they depart significantly from ODP conventions. This section applies only to sedimentary deposits recovered; procedures followed for description of igneous and metamorphic rocks recovered during Leg 183 are described in "Physical Volcanology," "Igneous Petrology," "Alteration and Weathering," and "Structural Geology".

Visual Core Description

Information from megascopic description of each core was recorded manually for each core section on paper VCD forms. This information was then condensed and entered into AppleCORE (version 0.7.5g) software, which generates a simplified, one-page graphical description ("barrel sheet") of each core. Barrel sheets are presented with whole-core images (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). Grain-size divisions for siliciclastic sand, silt, and clay are those of Wentworth (1922) (see "Sediment Classification"). The siliciclastic fraction is represented by a single "Siliciclastic Sediment" symbol if it is homogeneous in texture, or by two vertical strips of these symbols if texturally distinct siliciclastic sediments are interbedded (e.g., interbeds of sand and clay). Alternatively, in some cases, where each interval of interbedded lithologies can be graphically displayed, these lithologies are shown by horizontal strips of the two siliciclastic symbols.

Sediment color was generally determined visually; in most cases the Munsell Soil Color Charts (1971) were used to give a more precise color to a sediment interval. A wide variety of features that characterize the sediment, such as bed thicknesses, primary sedimentary structures, bioturbation parameters, soft-sediment deformation, and structural and diagenetic features 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 detailed section-by-section paper VCDs can be obtained from ODP. A key to the full set of symbols used on the graphic sedimentologic columns is shown in Figure F3. Bed thickness is characterized by the terms "very thick bedded" (>100 cm thick), "thick bedded" (30-100 cm thick), "medium bedded" (10-30 cm thick), "thin bedded" (3-10 cm thick), and "very thin bedded" (1-3 cm thick) (McKee and Weir, 1953).

Deformation and disturbance of sediment that clearly resulted from the coring process are illustrated in the "Drilling Disturbance" column, using symbols shown in Figure F3. Blank regions indicate the absence of coring disturbance. Detailed accounts of drilling disturbance appear in many previous ODP volumes (e.g., Leg 155, Shipboard Scientific Party, 1995). Locations of all samples taken for shipboard analysis are indicated in the "Samples" column by the codes listed in Figure F3.

A summary lithologic description with sedimentologic highlights is given in the "Remarks" column of the barrel sheet. This generally gives all the major sediment lithologies; important minor lithologies; an extended summary description of the sediments, including color, composition, sedimentary structures, trace fossils identified, extent of bioturbation, and other notable characteristics; and age of the sediments as determined by shipboard paleontologists and paleomagnetists. Descriptions and locations of thin, interbedded, or minor lithologies that could not be depicted in the "Graphic Lithology" column are presented in the "Remarks" column, where space permits.

Sediment Classification

The sediment classification scheme used during Leg 183 is descriptive and follows the ODP classification (Mazzullo et al., 1988), with some simplifying modifications for mixed siliclastic and biogenic sediments (Fig. F4). Classification is based primarily on megascopic description of the cores and examination of smear slides. During Leg 183, total calcium carbonate content of the sediments determined on board (see "Organic and Inorganic Geochemistry") was also used to aid in classification. Composition and texture are the only criteria used to define lithology. Genetic terms (such as pelagic, neritic, hemipelagic, debris flow, and so forth) 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 mm in size. Biogenic components are not described in textural terms. Thus, a sediment with 55% sand-sized foraminifers and 45% siliciclastic clay is called a foraminifer clay, not a foraminifer clayey sand. Similar considerations apply to sediment containing abundant sand-sized glauconite.

The principal name applied to a sediment is determined by the component or group of components (e.g., total biogenic carbonate) that comprise(s) >60% of the sediment or rock, except for subequal mixtures of biogenic and siliciclastic material. If the total of a siliciclastic component is >40%, 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, silty clay, sandy mud, or sand. However, if the total of biogenic components is >60% (i.e., siliciclastic material <40%), then the principal name applied is "ooze" (Fig. F4B).

In mixtures of biogenic and nonbiogenic material, where the biogenic content is 30%-60% (termed "mixed sediments" in the ODP classification), the name consists of two parts: (1) the 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 siliciclasti). If any component (biogenic or siliciclastic) represents only 10%-30% of a sediment, it qualifies for minor modifier status and is hyphenated with the word "-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).c components (e.g., foraminiferal clay) (Fig. F4B). If any component (biogenic or siliciclastic) represents only 10%30% of a sediment, it qualifies for minor modifier status and is hyphenated with the word "-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:
foraminifer-bearing nannofossil clay
(11%) (34%) (55%)
diatom-bearing foraminifer ooze
(20%) (80%)

Chemical sediments and diagenetic beds or nodules, including minerals formed by inorganic precipitation such as evaporites and many carbonates (e.g., packstone, rudstone, and grainstone) are classified according to mineralogy, texture, and fabric following the ODP classification (Mazzullo et al., 1988). For semilithified to lithified sediments, the suffix "-stone" is added to the principal names sand, silt, clay, or mud. The term "chalk" is used for partially lithified sediment composed of >60% calcareous nannofossils. Sediments composed of >60% siliciclastic and volcanic grains are classified as "volcaniclastic" if they contain a higher proportion of volcaniclastic than siliciclastic grains. The classification used for volcaniclastic sediments is described in "Physical Volcanology".

X-Ray Diffraction

Relative abundances of the main silicate and carbonate minerals were determined semiquantitatively using a Philips model PW-1729 X-ray diffractometer with Cu K radiation (Ni filter). 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, and count time 0.5 s. Peak intensities were converted to values appropriate for a fixed-slit width. An interactive software package (MacDiff 3.3.0 PPC) was used on a Macintosh computer to identify the main minerals. Most diffractograms were peak corrected to match the main calcite peak at 3.035 , except where quartz was present as a major component (major peak at 3.343 ). In absence of both quartz and calcite, no peak correction was applied. Identifications were based on multiple peak matches, using the mineral database provided with MacDiff (Table T2).

Peak areas were measured only to estimate calcite/dolomite ratios. In all other cases, minerals were grouped as "major" or "trace" components depending on relative peak heights. It is not possible to reasonably estimate the proportions of clay minerals, glass, or amorphous opal with the bulk samples used for analysis; however, mixtures of these minerals with calcite comprise most of the analyzed samples. Relative abundances reported in this volume during Leg 183 are useful for general characterization of the sediments, but they are not precise quantitative data.

Smear Slides

Petrographic analysis of the sediment was primarily by smear-slide description. See the "Core Descriptions" contents list for tables summarizing data from the smear slides. 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 because 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 incorporated into the smear evenly. 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 "Organic and Inorganic Geochemistry").

Spectrophotometer

Reflectance of visible light from cores was routinely measured downhole using a Minolta Spectrophotometer (Model CM-2002) mounted on the archive multisensor track (AMST) (see "Physical Properties"). 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 downhole for visible wavelengths (400-700 nm). Spectrophotometer readings were taken before 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; in press). During Leg 183, there was no way to program the AMST software to avoid taking measurements in void intervals in the cores or in disturbed areas of core with drilling slurry or biscuits. This produced many spurious measurements, which must be edited from the color data before use.

Warning: During the cruise and postcruise, it was determined that a large number, if not all, values measured with the spectrophotometer are in error. Whether these data can be restored to correct values or rendered otherwise usable is being assessed by ODP personnel. Users of these data should check with ODP database personnel before proceeding.

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