In each chapter of the Leg 202 Initial Reports volume, we have combined the sedimentology and physical properties measurements, observations, and interpretations into a single section titled "Lithostratigraphy." The purpose of integrating these two traditionally distinct sections is twofold. First, physical properties measurements are often used in the interpretation of lithologic units. Second, lithology largely controls the measured physical parameters. Thus, a combined subchapter serves to integrate complementary measurements and observations while decreasing the redundancy in reporting.
Although all sedimentological and physical properties results are reported together, the two sections were left separate in this chapter because of the distinctly different techniques and methodologies related to each. Lithologic units are defined based primarily on the sedimentological divisions. Physical properties data for lithologies within each unit will then be included and discussed. The benefit of this approach is to minimize ambiguity about the significance of divisions based on downhole contrasts in lithology and physical properties measurements at each site.
Lithologic names consist of a principal name based on composition, degree of lithification, and/or texture as determined from visual description and smear slide observations. For a mixture of components, the principal name is preceded by major modifiers (in order of increasing abundance) that refer to components making up >25% of the sediment (Fig. F1). Minor components that represent between 10% and 25% of the sediment precede the principal name and the hyphenated term "-bearing" in order of increasing abundance. In the case of pyrite, micrite, or other authigenic components, the 10%-25% abundance range would be acknowledged by using the suffix "with." For example, an unconsolidated sediment containing 45% nannofossils, 30% clay minerals, 10% micrite, 10% foraminifers, and 5% feldspar would be described as a foraminifer-bearing clayey nannofossil ooze with micrite. Although sedimentary components ranging from 10%-25% are reflected in the sediment name in the description column as "-bearing," these components are not designated in the graphic lithology column. These naming conventions follow the ODP sediment classification scheme of Mazzullo et al. (1988) with the exception that during Leg 202 a separate "mixed sediment" category was not distinguished. Chemical sediments generally were not encountered during Leg 202 except as accessory minerals and nodules, and therefore, this category is not addressed below. Micrite, which may be formed by local recrystallization or by precipitation from solution, was frequently encountered during Leg 202 and is included below within the discussion of nonpelagic carbonates and authigenic minerals.
Granular sediments were subdivided on the basis of composition and abundance of different grain types estimated from visual examination of the core, smear slides, thin sections, and by shipboard measurements of carbonate content (see "Sedimentary Inorganic Carbon and Organic Carbon, Nitrogen, and Sulfur Concentrations") and shipboard X-ray diffraction (XRD) analyses. Sediments containing >50% silt- and sand-sized volcanic grains were classified as ash layers. Larger volcanic clasts (breccia) were not encountered except as fragments of basalt at or near the base of drilled sections. Size divisions for grains are those of Wentworth (1922) (Fig. F2). Size-textural qualifiers were not used for pelagic sediment names (e.g., nannofossil clay implies that the dominant component is detrital clay, rather than clay-sized nannofossils).
Grain types in granular sediments and rocks were classified according to depositional origin and mineralogy: (1) pelagic calcareous, (2) pelagic siliceous, (3) nonpelagic calcareous, and (4) siliciclastic particles. Their definitions are as follows:
Variations in the relative proportions of the five grain types described above define four major classes of granular sediments and rocks: calcareous and siliceous pelagic, nonpelagic calcareous, siliciclastic, and mixed.
Pelagic sediments and rocks contain 60% or more pelagic grains and 40% or less nonpelagic calcareous and siliciclastic grains. Nonpelagic calcareous sediments and rocks include 60% or more nonpelagic calcareous grains and 40% or less pelagic plus nonpelagic siliciclastic grains. Siliciclastic sediments and rocks are composed of 60% or more siliciclastic grains and 40% or less pelagic plus nonpelagic calcareous grains. For mixtures of pelagic and nonpelagic calcareous grains, we used the classification scheme of Dunham (1962) (Fig. F3). For mixtures of siliciclastic, biogenic, and other sediments, we followed the procedures outlined in Table T2.
Sediments and rocks were named solely on the basis of composition and texture. Within each class, granular sediments and rocks were classified using a principal name and major and minor modifiers. Principal names define the degree of consolidation (firmness) and granular sediment class. Composition is the most important classifier for pelagic and siliciclastic sediments and rocks (Table T2), whereas texture is more significant for the classification of nonpelagic calcareous sediments and rocks (Fig. F3).
Compositions and textures of cored sediments and rocks were determined on board ship by (1) unaided visual observation, (2) visual observation using a hand lens, and (3) visual estimates in smear slides, and coarse fractions were verified with the aid of a microscope. Calcium carbonate content was estimated qualitatively in smear slides and quantitatively by coulometric analysis (see "Geochemistry"). Selected samples were also examined in thin section and by XRD.
Firmness of recovered materials was defined as in Gealy (1971). Three classes of firmness were used to describe calcareous sediments and rocks:
Two classes of firmness were used to describe siliciclastic sediments and rocks:
Principal names used to describe pelagic sediments and rocks from Leg 202 are as follows:
Nonpelagic calcareous sediments and rocks were classified using a modification of the original Dunham (1962) classification in conjunction with depositional textures described by Embry and Klovan (1971) (Fig. F3):
Sediments and rocks that consist of a mixture of pelagic and nonpelagic calcareous grains and contain aragonite and/or magnesian calcite (confirmed by XRD analysis) were (1) described lithologically using the classification scheme explained above for nonpelagic calcareous sediments and rocks and (2) labeled additionally as periplatform oozes and periplatform chalks (e.g., Schlager and James, 1978).
For siliciclastic sediments and rocks, texture is the main criterion for the selection of a principal name. The Udden-Wentworth grain-size scale (Wentworth, 1922) (Fig. F2) defines the grain-size ranges and the names of the textural groups (gravel, sand, silt, and clay) and subgroups (fine sand, coarse silt, etc.). When two or more textural groups or subgroups are present, the principal names appear in order of increasing abundance. Eight major textural categories can be defined on the basis of the relative proportions of sand, silt, and clay (Fig. F4). Distinguishing between some size categories is difficult (e.g., silty clay and clayey silt) without accurate measurements of weight percentages. The terms "conglomerate" and "breccia" are the principal names of gravels with well-rounded and angular clasts, respectively.
To describe the lithology of the granular sediments and rocks in greater detail, the principal name of a granular-sediment class is preceded by major modifiers and followed by minor modifiers (Table T2). Minor modifiers are preceded by the term "-bearing." The most common uses of major and minor modifiers are to describe the composition and textures of grain types that are present in major (25%-40%) and minor (10%-25%) proportions in addition to the principal sediment. In addition, major modifiers can be used to describe grain fabric, grain shape, and sediment color. The composition of pelagic grains can be described in greater detail with the major and minor modifiers "nannofossil," "foraminifer," "calcareous, and siliceous." The terms "calcareous" and "siliceous" are used to describe sediments that are composed of calcareous or siliceous pelagic grains of uncertain origin. The compositional terms for nonpelagic calcareous grains include the following major and minor modifiers as skeletal and nonskeletal grains:
The textural designations for siliciclastic grains utilize standard major and minor modifiers, such as "gravel(-ly)," "sand(-y)," "silt(-y)," and "clay(-ey)." The character of siliciclastic grains can be described further by mineralogy using modifiers such as "quartz," "feldspar," "glauconite," "mica," "lithic," or "calcareous."
The standard method of splitting cores by pulling a wire lengthwise through the center tends to smear the cut surface of soft sediments and obscure fine details of lithology and sedimentary structure. When necessary during Leg 202, specific intervals of the archive halves of cores were carefully scraped with a stainless steel or glass scraper to prepare the surface for unobscured sedimentologic examination and digital imaging. Scraping was done parallel to bedding with a freshly cleaned tool to prevent cross-stratigraphic contamination.
Detailed sedimentologic observations and descriptions were recorded by hand for individual cores using handwritten visual core description (VCD) sheets. These observations were synthesized for each core in the computer-formatted sediment core description forms (Fig. F5) (AppleCORE "barrel sheets"). Exceptions to the standard ODP conventions adopted by the Leg 202 Scientific Party are described in the following sections.
The lithologic description that appears on each core description form (barrel sheet) consists of two parts: (1) a heading that lists the major sediment lithologies observed in the core and (2) a more detailed description of the sediments, including location in the core of significant features. Descriptions and locations of thin, interbedded, or minor lithologies, color, samples, coring conditions, and so on, are included in the text.
Sediment types determined from the above classification scheme are represented graphically in the "Graphic Lithology" column of the barrel sheets using the symbols illustrated in Figure F6. A maximum of three different lithologies (for interbedded sediments) or three different components (for mixed sediments) can be represented within the same core interval. Minor lithologies present as thin interbeds within the major lithology are shown by a dashed vertical line dividing the lithologies. Percentages are rounded to the nearest 10%, and lithologies that constitute <10% of the core are generally not shown but are listed in the "Lithologic Description" section. In some cases (e.g., distinctive ash layers) individual occurrences of minor lithologies are included graphically in the lithology column. Intervals that are a few centimeters or greater in thickness can be portrayed accurately in this column. Contact types (e.g., sharp, scoured, and gradational) are also shown within the "Graphic Lithology" column.
Visible bioturbation was classified into five intensity levels based on the degree of disturbance of the physical sedimentary structures (Fig. F6):
These categories are based on the five ichnofossil indices (ii1-ii5) of Droser and Bottjer (1986) and are illustrated with graphic symbols in the "Bioturbation" column. Visual recognition of bioturbation was often limited in homogeneous sediments, particularly in very dark siliciclastic clays and white calcareous oozes.
The locations and types of sedimentary structures visible on the prepared surface of the archive half of cores are shown in the "Structure" column of the core description form. The column is divided into three vertical areas for symbols. The symbols on the left side of the "Structure" column indicate the bedding characteristics (including color banding) of the sediment. We followed a slightly amended version of McKee and Weir (1953) to distinguish thicknesses of bedding units, whether identified by composition, texture, or color. Stratigraphic units are very thick bedded (or banded) if >100 cm in thickness, thick bedded (30-100 cm), medium bedded (10-30 cm), and thin bedded (<10 cm) in thickness. Finer millimeter-scale layers were described as laminae, although they were only rarely encountered during Leg 202. The abundance of visually detectable bioturbation is shown in the central portion of the "Structure" column of the barrel sheet as described above. The symbols on the left side of the "Structure" column indicate the location of individual bedding features and any other sedimentary features such as scours and ash layers, ripple laminations, ichnofossils, or shell fragments. Lithologic accessories in the center portion include diagenetic features such as nodules and sulfides. Fossils, shell fragments, and ichnofossils are included on the right side. The symbols used to designate the structures found in Leg 202 are shown in Figure F6.
Drilling-related sediment disturbance that persists over intervals of ~20 cm or more is recorded in the "Disturbance" column using the symbols shown in Figure F6. The degree of drilling disturbance is described for soft and firm sediments using the following categories:
The degree of fracturing in indurated sediments and igneous rocks is described using the following categories:
The positions of samples taken from each core for analysis are indicated by letters in the "Sample" column of the core description form as follows: SS (smear slide), PAL (micropaleontology), CAR (carbonate), PP (physical properties), XRD (X-ray diffraction), THS (thin section), and IW (interstitial water). In most cases, samples were coordinated to provide data from the same horizon. Typically, CAR and PP samples were taken at 74-76 cm in each section of cores from the first long hole at each site, typically hole "A." Smear slides were also generally taken at 75 cm in Sections 1 and 3 in the archive half of these cores, and additional SS samples were taken in order to capture the range of lithologies present.
The color of the sediments was determined qualitatively using Munsell Color Company (1994) soil color charts and rock color charts (Rock-Color Chart Committee, 1991). When portions of the split core surface required cleaning with a stainless steel or glass scraper, this was done prior to color assessment. The assigned colors were then entered in the "remarks" field of the barrel sheet.
Automated measurements using the Minolta spectrophotometer provided quantitative color measurements (see "Physical Properties"). Observations of the damp core surface were made as soon as possible after the core was split. Note that many surficial oxidation reactions are completed within seconds after splitting organic-rich cores of reduced sediment, and others proceed over the course of 1-2 hr. The surface colors documented here are not necessarily identical to those of the pristine, unoxidized sediment prior to the exposure to atmospheric oxygen, or to the stable "equilibrium" colors reached after complete surface oxidation.
Smear slide samples were taken from the archive halves. For each sample, a small amount of sediment was removed with a wooden toothpick, dispersed evenly in deionized water on a 1-in x 3-in glass slide, and dried on a hot plate at a low setting. A drop of mounting medium and a 1-in x 1-in cover glass was added, and the slide was placed in an ultraviolet light box for ~30 min. Once fixed, each slide was scanned at 100x-200x with a transmitted light petrographic microscope using an eyepiece micrometer to assess grain-size distributions in clay (<4 µm), silt (4-63 µm), and sand (>63 µm) fractions. The eyepiece micrometer was calibrated once for each magnification and combination of ocular and objective, using an inscribed stage micrometer.
The volume percent technique (Fig. F7) was often employed as an intermediate step toward calculating relative proportions of each grain size and type. Note that smear slide analyses tend to underestimate the abundance of sand-sized and larger grains (e.g., foraminifers, radiolarians, and siliciclastic sand) because these are difficult to incorporate into smear. At the same time, clay-sized biosilica, being transparent and isotropic, are very difficult to quantify. Clay minerals, micrite, and nannofossils can also be difficult to distinguish at the very finest (<~4mm) size range. After scanning for grain-size distribution, several fields were examined at 200x-500x for mineralogical and microfossil identification. Standard petrographic techniques were employed to identify the commonly occurring minerals and biogenic groups, as well as important accessory minerals and microfossils.
Smear slide analysis data tables are included in this volume. 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 percentages of sand, silt, and clay, together with all identified components.