LITHOSTRATIGRAPHY

Barrel Sheets

Information from macroscopic and microscopic examination of each core section was recorded by hand on a visual core-description (VCD) form. This information was condensed and entered into the AppleCORE (v8.1b) program to generate simplified graphic "barrel sheets" (see the "Core Descriptions" contents list). Site, hole, and mbsf interval are given at the top of the barrel sheet, with mbsf positions of core sections along the left margin. Copies of the original VCD sheets, which may contain additional core description, are available from ODP by request. Barrel sheet columns are discussed below, followed by an outline of the lithostratigraphic classification used for Leg 189.

Graphic Descriptions

The key for lithologic and contact symbols is presented in Figure F3. Lithologic symbols are arranged within the barrel sheet lithologic column in order of their relative abundance from left to right: minor lithologic modifiers (10%-25% in smear slide) are represented by 20%, major lithologic modifiers (>25%-50% in smear slide) by 30%, and the primary lithology by the remaining 50%-100%. If two minor modifiers are present, each is represented by 10% within the lithologic column, and if two major modifiers are present, each is represented by 15%. Lithologic contacts ranged from gradational (hashed thick line) to sharp (solid thin line).

Keys for sedimentary structures, lithologic accessories, ichnofossils, and fossils are presented in Figure F3. Isolated occurrences of these features are represented by a single symbol, whereas multiple to continuous occurrences are indicated by a vertical line with a centered symbol.

Bioturbation

Visible bioturbation was classified into five intensity levels based on the degree of disturbance of the physical sedimentary structures (Fig. F3):

  1. Absent: No bioturbation; all physical sedimentary structures are preserved.
  2. Rare: Isolated trace fossils; up to 10% of physical sedimentary structures are disrupted.
  3. Present: ~10%-40% disrupted physical sedimentary structures; burrows are generally isolated but locally overlap.
  4. Common: ~40%-60% bioturbated; last vestiges of physical sedimentary structures only are discernible.
  5. Abundant: Bedding completely disturbed; burrows are still discrete in places.

These categories are based on the five ichnofossil indices (ii1-ii5) of Droser and Bottjer (1986). Visual recognition of bioturbation was often limited in homogeneous sediments, particularly white oozes; X-radiographic examination is warranted for such sediments.

Drilling Disturbance

Drilling disturbance of relatively soft sediments (i.e., wherein intergrain motion was possible) was classified into five categories:

  1. Slightly disturbed: Bedding contacts are slightly bent.
  2. Moderately disturbed: Bedding contacts are extremely bowed.
  3. Very disturbed: Bedding is completely deformed and may show diapiric or minor flow structures.
  4. Flow-in: Sediments are completely disturbed with pervasive flow structures.
  5. Soupy: Sediments are water saturated and show no traces of original bedding or structure.

To aid in evaluation of physical properties and other data, flow-in disturbance for each core beginning with Hole 1170A was rated on a qualitative scale as follows:

F0 = undisturbed sediments with only minor "edge effect."
F1 = upward bowing of about one-third core width (~2.2 cm vertical stretching) or 30° inclination of sediments.
F2 = extreme upward bowing of about one-third to one-half core width (~2.2-3.4 cm vertical stretching) or 30°-60° inclination.
F3 = onset of minor flow-in beyond that of F2.
F4 = massive flow-in.

Drilling disturbance of lithified sediments (i.e., wherein intergrain motion was not likely because of compaction, cementation, etc.) was classified into three categories:

  1. Fractured: Core pieces are in situ or partly displaced, but original orientation is preserved or recognizable; pieces may be supported by drill slurry or breccia.
  2. Biscuit: Core pieces are from the cored interval and probably in correct stratigraphic sequence (although they may not represent the entire section), but original orientation is lost. Pieces are supported in drill mud.
  3. Breccia: Core pieces have completely lost their original orientation and stratigraphic position.

To aid in evaluation of physical properties and other data, a qualitative disturbance rating was developed for biscuited and fractured lithified sediments to distinguish the amount of undisturbed "coherent" sediment from intercalated drill slurry and breccia, beginning with Hole 1170A. For each core section, the percentage of biscuits along the core center was estimated and rated as follows:

B0 = >98% of undisturbed sediment.
B1 = 97%-91% of undisturbed sediment.
B2 = 90%-85% of undisturbed sediment.
B3 = 84%-70% of undisturbed sediment.
B4 = <70% of undisturbed sediment.

Samples

The positions of core samples are indicated in the sample column as follows: SS (smear slide), IW (interstitial water), PAL (micropaleontology), XRD (X-ray diffraction analysis), and TS (thin section). Contextual information includes sample location, whether the smear-slide or thin-section sample represents a dominant (D) or minor (M) lithology, and the estimated percentages of different grains.

Color

Reflectance of visible light was measured at 2-cm intervals for all core material using a Minolta Spectrophotometer (Model CM-2002) mounted on the archive multisensor track (AMST). The spectrophotometer was calibrated to zero once per shift. Regardless of sediment condition, archive-core measurements were made after the surface was saturated with water and covered with Gladwrap brand clear plastic wrap. This procedure protected the spectrophotometer lens and minimized variance caused by the degree of sediment wetness.

The measured visible light spectra provided a high-resolution stratigraphic record of color variations from 31 separate determinations of reflectance in 10-nm-wide spectral bands from 400 to 700 nm. These high-resolution data as well as high-precision Munsell color values measured at the same time are archived in the Janus database. Note that the precision of these values should not be confused with their accuracy in capturing stratigraphic patterns; core disturbances, particularly biscuiting and flow-in, require that such data be carefully evaluated against core photos and disturbance descriptions. Only major Munsell color changes, as determined by eye, using color charts, are reported in the barrel sheets. Additional information about measurement and interpretation of spectral data with the Minolta spectrophotometer can be found in Balsam et al. (1997, 1998, 1999).

Description

The lithologic descriptions list the major lithology in capital letters, followed by descriptions of major and minor lithologies, and, finally, a description of other significant core features and their locations.

Lithologic Classification

The lithologic classification scheme for Leg 189 is based on three end-member components (i.e., biogenic siliceous, biogenic calcareous, and terrigenous siliciclastic), the texture of siliciclastics (i.e., clay-, silt-, sand-sized grains), and the degree of sediment induration (Fig. F4). Percentages of the different lithologic components, as well as siliciclastic textural percentages, were determined by smear-slide examination. These percentages defined the preliminary lithologies presented on the accompanying barrel sheets. Typically, smear slide-based estimates of percent carbonate often differed somewhat from subsequent coulometric-based measurements of percent carbonate. Where this carbonate offset produced different classifications for the major lithologic units within site summaries, the coulometric-based classification was preferred. Note that barrel sheet lithologies are based exclusively on smear slides and core observations available at the time of description and do not include coulometric results. Each compositional end-member of the lithologic classification scheme is discussed in detail below and related graphically to one another in Figure F4.

Calcareous Biogenic

Calcareous biogenic lithologies are composed of >50% biogenous grains of which >50% are calcareous (Fig. F4A). Major (>25%-50%) and minor (10%-25%) modifiers describe the nature of the calcareous biogenic grains, as well as the composition of accessory siliceous biogenic or siliciclastic grains. Calcareous biogenic grains are described by the terms "foraminifer," "nannofossil," and "calcareous" (for unidentifiable grains), followed by the suffix "-bearing" where the component is present in minor amounts.

If the sediment was soft (i.e., readily deformed under the pressure of a finger or spatula blade and cut by the wire cutter), it was termed an ooze. If the sediment was firm (i.e., easily scratched by fingernail or edge of a spatula and cut by band or diamond saw), the term ooze was replaced by the term chalk. If the sediment was hard (i.e., not scratched by fingernail or edge of a spatula and cut by band or diamond saw), the term ooze was replaced by the term limestone. Additional modifiers for the texture of terrigenous sediments and the composition of accessory siliceous biogenic sediments were described using terms from the sections below (see "Siliceous Biogenic" and "Siliciclastic Sediments"). For example, a firm sediment composed of 5% foraminifers, 15% siliciclastic silt, 18% diatoms, and 60% nannofossils would be termed a silt- and diatom-bearing nannofossil chalk. Note that multiple minor or major modifiers are listed in order of increasing predominance.

Siliceous Biogenic

Siliceous biogenic sediments are composed of >50% biogenous grains of which >50% are siliceous (Fig. F4A). Major and minor modifiers describe the composition of the siliceous biogenic grains, as well as the composition of accessory carbonate biogenic and siliciclastic grains. The composition of siliceous biogenic grains is described based on their origin using the terms "radiolarian," "diatom," "silicoflagellate," and "siliceous" (for unidentifiable siliceous biogenic debris), followed by the suffix "-bearing" when a component is present in minor amounts. If the sediment was soft (i.e., readily deformed under the pressure of a finger or spatula blade and cut by the wire cutter), it was termed an ooze. If the sediment was firm (i.e., easily scratched by fingernail or edge of a spatula and cut by band or diamond saw), the term ooze was replaced by the term chalk. If the sediment was hard (i.e., not scratched by fingernail or edge of a spatula and cut by band or diamond saw), the term ooze was replaced by the term diatomite or radiolamite, as appropriate. Modifiers for the texture of siliciclastic sediments are described using terms from "Siliciclastic Sediments" the composition of accessory calcareous biogenic sediments is described using terms from "Calcareous Biogenic". For example, a soft sediment composed of 8% siliciclastic clay, 20% nannofossils, 35% radiolarians, and 37% diatoms would be termed a nannofossil-bearing radiolarian diatom ooze.

Siliciclastic Sediments

Siliciclastic sediments are composed of >50% siliciclastic grains and are classified according to the grain-size textures of clay (<3.9 µm), silt (3.9-6.25 µm), and sand (>6.25 µm-2.0 mm). The percentages of these three grain-size textures defined the major lithology based on the right ternary diagram in Figure F4B. Where the sediment was indurated (i.e., not easily scratched by fingernail or spatula edge), the suffix "-stone" was added (e.g., claystone, siltstone, and sandstone).

The predominant components of siliciclastic grains are described by major modifier terms such as "sand," "silt," or "clay" where any terms account for >25% of the siliciclastic percentage. Where biogenic grains are present in minor amounts (10%-25%) within siliciclastic it is followed by the suffix "-bearing." Where a biogenic grain comprises >25%-50% of the total sediment, the major modifier of "siliceous" or "calcareous" (or more precise terms such as "radiolarian," "foraminifer," etc.; see above) is applied.

Three examples are presented to illustrate this siliciclastic classification. A sediment composed of 15% foraminifers, 25% glauconite, and 60% sand would be termed a foraminifer-bearing glauconitic sand. A sediment composed of 65% gravel and 35% silt would be termed a silty gravel. A sediment composed of 20% volcanic ash, 30% silt, and 50% sand would be termed an ash-bearing silty sand.

Clay Mineralogy

The purpose of this study was to recognize the major variations that occurred in the paleoenvironments, as expressed by the changing nature and abundance of clay minerals, using a sampling interval of one sample every two cores.

Methodology

The samples were decalcified using 10% acetic acid, then washed repeatedly with deionized water in a centrifuge. The carbonate-free fraction was deflocculated with a 1% Calgon (sodium hexametaphosphate) solution and homogenized in a sonic dismembrator for 1 min. The clay fraction (<2 µm) was separated by centrifugation, and the clay residue was then deposited onto glass slides and dried in an oven. Three separate X-ray analyses were run on each of the samples at a scan speed of 1° 2/min using CuK radiation, a Ni filter, and a monochromator. These were (1) from 2° to 32° 2 on a glass slide; (2) from 2° to 16° 2 on a slide saturated with ethylene glycol for 12 hr in an oven; and (3) on a slide heated to 550°C for 1 hr. Percentage evaluations calculated using MacDiff software were based on peak areas. The clay-mineral indexes smectite/smectite+illite (S/S+I), kaolinite/kaolinite+illite (K/K+I), and kaolinite/kaolinite+smectite (K/K+S) were derived from the ratios of the 001 smectite, illite, and kaolinite peak intensities obtained from the glycolated samples.

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