doi:10.2204/iodp.proc.301.105.2005

Lithostratigraphy

Lithologic classification

Expedition 301 lithologic classification is based on three end-member grain components (biogenic silica, carbonate, and terrigenous or volcanic grains), the grain size of the terrigenous component (i.e., proportions of clay, silt, and sand), and the degree of sediment induration (e.g., chalk versus limestone). Percentages of end-member grain components, as well as siliciclastic textures used to define lithologies reported on the visual core descriptions (VCDs) and barrel sheets, were determined with a hand lens, binocular microscope, or smear slide examination of disaggregated core material. Thin sections and carbonate analyses were used to refine or modify lithology designations made at the description table.

In detail, 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% or more of the sediment. Minor components that represent between 10% and 25% of the sediment follow the principal name after a "with" in order of increasing abundance. Thus, an unconsolidated sediment containing 30% nannofossils, 25% clay minerals, 20% foraminifers, 15% quartz silt, and 10% manganese nodules would be described as a clayey nannofossil ooze with manganese nodules, quartz silt, and foraminifers. These naming conventions follow the Ocean Drilling Program (ODP) sediment classification scheme (Mazzullo et al., 1988), with the exception that during Expedition 301 a separate "mixed sediment" category was not distinguished.

Sediment was classified on the basis of composition estimated by visual examination of the core and smear slides, by shipboard measurements of carbonate content, and by shipboard X-ray diffraction (XRD) analyses. 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).

Terms that describe lithification vary depending upon the dominant composition:

• Sediment derived predominantly from calcareous pelagic organisms (e.g., calcareous nannofossils and foraminifers):

• Ooze = sediment can be deformed with a finger.

• Chalk = sediment can be scratched easily by a fingernail.

• Limestone = sediment cannot be scratched easily.

• Sediment derived predominantly from siliceous microfossils (diatoms, radiolarians, and siliceous sponge spicules):

• Ooze = sediment can be deformed with a finger.

• Radiolarite/spiculite/diatomite = sediment cannot be easily deformed manually.

• Porcellanite = siliceous limestone/claystone that has a dull luster and is less hard and compact than chert (Keene, 1975). It may contain a mix of opal, quartz, clay minerals, and carbonate. Note that the terms porcellanite and chert do not imply crystallinity of the silica.

• Chert = sediment displays a glassy luster. It may contain a mix of opal, quartz, clay minerals, and carbonate.

• Sediment derived predominantly from siliciclastic material: If the sediment can be deformed easily with a finger, no lithification term is added and the sediment is named for the dominant grain size. For more consolidated material, the lithification suffix "-stone" is appended to the dominant size classification (e.g., clay versus claystone).

• Sediment composed of sand-sized volcaniclastic grains:

• Ash = sediment can be deformed easily with a finger.

• Tuff = more consolidated material.

• Lapilli = coarse-grained material.

Visual core description and barrel sheets

Shipboard scientists were responsible for visual core description and smear slide analysis. Detailed observations of each section were recorded initially by hand on a blank sediment VCD form. This information was subsequently entered into the AppleCORE software (version 9.4a), which generates a simplified, annotated graphical description (barrel sheet) for each core (Fig. F3). These barrel sheets are linked to corresponding core photographs in "Core Descriptions."

Site, hole, and depth in mbsf are given at the top of the barrel sheet, with depth positions of core sections indicated along the left margin. Columns on the barrel sheets include Graphic Lithology, Bioturbation, Sedimentary Structures, Fossils (ichnofossils), Sediment Disturbance, Sample Types, Color, and Description. These columns are discussed below, followed by an outline of the lithostratigraphic classification used during Expedition 301.

Graphic lithology

Lithologies of the core intervals recovered are represented on barrel sheets by graphic patterns in the Graphic Lithology column (Fig. F4). For intervals containing homogeneous mixtures of multiple lithologies, symbols are arranged within the column from left to right in order of their relative abundance. Graphic lithologies are used for components that compose 10% or more of the total sediment, with only the three most abundant components shown. The width of each pattern in the column approximates the relative abundance of that component. Relative abundances reported in this column are useful for general characterization of the sediment, but they are not precise, quantitative data. No graphic lithology is shown for intervals from which whole round samples were taken.

Sedimentary structures

Sedimentary structures formed by natural processes and not as a result of drilling disturbance are represented on the barrel sheet in the Structure column (Fig. F4). Structures formed by both biogenic and physical processes are included. These include varying degrees of bioturbation, types of trace fossils, parallel laminations, and soft-sediment deformation structures.

Ichnofossils

Symbols are used to denote the location of clearly identifiable ichnofossils (Fig. F4).

Bioturbation

The extent of general bioturbation is indicated in the Bioturbation column (Fig. F4). Bioturbation is shown by the shading of a vertical bar to the right of the Graphic Lithology column. Using a scheme similar to that proposed by Droser and Bottjer (1986), five levels of bioturbation were recognized. Bioturbation intensity is classified as follows:

• 5 = abundant (>75%)

• 4 = common (50%–75%)

• 3 = moderate (10%–50%)

• 2 = rare (<10%)

• 1 = barren (none)

Color

Color is determined qualitatively using the Munsell rock color charts (Rock-Color Chart Committee, 1991) and is described immediately after the cores are split to avoid color changes associated with drying and oxidation. Color is generalized in the Color column with abbreviations (e.g., "dk mo Br," which is dark mottled brown) (Table T1).

Drilling disturbance

Symbols are used to denote sediment disturbance induced by the coring process (Fig. F4).

• Slightly fractured: Core pieces are in situ with cracks across the core each few centimeters. Some pieces may not be in stratigraphic continuity with the adjacent piece where core recovery was incomplete.

• Moderately fractured: Core pieces are probably in correct stratigraphic sequence but may not represent the entire section. The core is more fractured than in the preceding category.

• Highly fractured: Core pieces are probably in correct stratigraphic sequence but are strongly fractured and may be rotated. Such intervals are difficult to describe because the fracturing obscures primary features.

Symbols are positioned at the location in the section where that feature is observed. If the feature extends over an interval, the symbol appears centered on a vertical line to denote the stratigraphic extent of occurrence.

Sample types

Sample material taken for shipboard analysis consisted of IW from whole-round samples; whole-round samples for microbiology (WRB), organic geochemistry (WRO), and physical property analyses (WRP); smear slide "toothpick" samples (SS); discrete samples for XRD; and carbonate analysis (CAR). The locations of other shipboard samples can be found by interrogation of the Janus database on the IODP Web site.

Description

The written description for each core contains a brief overview of major and minor lithologies that are present, as well as notable features (e.g., sedimentary structures).

Smear slide analyses

Smear slides can be prepared from moderately consolidated sedimentary rocks. For each smear slide, a small amount of archive-half sediment was gently crushed and dispersed in a dilute Calgon solution or deionized water on a 22 mm × 40 mm coverslip and then dried on a hot plate at a low setting. A drop of Norland optical adhesive was applied to a prelabeled 25 mm × 75 mm glass microscope slide, after which the coverslip was transferred onto the slide and cured in an ultraviolet light box. This procedure is different from most preparations in that the sediment dispersion is prepared on the coverslip rather than on the glass slide. The advantage is that small particles like nannofossils and clay minerals adhere directly to the coverslip and can be viewed at high magnification because they are very close to the top of the prepared slide. Some of the scientific party preferred to prepare smear slides directly on the more robust glass slide, otherwise following the procedure outlined above.

Smear slides were examined with a transmitted-light petrographic microscope equipped with a standard eyepiece micrometer to assess the percentages of different sized grains (sand, silt, and clay) and the proportions and presence of biogenic and mineral components.

In these tables, components are assigned to one of the following categories:

• T = trace (0%–2%)

• R = rare (2%–10%)

• M = minor (10%–20%)

• C = common (20%–40%)

• A = abundant (40%–60%)

• D = dominant (>60%)

Digital imaging

All archive core halves were scanned using a Geotek digital imaging system (DIS). The DIS uses an interference filter and three line-scan charge-coupled device arrays (1024 pixels each) to continuously record the three red-green-blue (RGB) color channels with 8 bit dynamic range. The standard DIS configuration produces 300 dpi on an 8 cm wide core with a zoom capability of up to 1200 dpi on a 2 cm wide core. Synchronization and track control is better than 0.02 mm, and a framestore card contains 48 MB of random access memory (RAM) for image acquisition. The camera aperture was set to maximize contrast within the lightest colored sediment of each core. Each archive section, along with a neutral gray color chip and identification bar-code label, was DIS scanned to produce a TIFF (no compression). Using the Geotek Image Tools utilities, these photos were resampled to produce a JPEG file with a resolution of ~300 dpi. The JPEG files were viewable via the Web browser as "photo table" composite images. Profiles for each RGB channel were produced by averaging pixels in 3 cm x 0.5 cm rectangles along the central axis of the core. The DIS system was calibrated for black and white whenever deemed necessary by IODP technical staff. This capability was essential for reviewing previous cores.

X-ray diffraction analyses

Routine samples for shipboard XRD analysis were taken from approximately three sections per core, and most were located adjacent to samples for analysis of methane concentration. Samples were freeze-dried, crushed either by hand or with a ball mill, and mounted as random bulk powders. The X-ray laboratory aboard the JOIDES Resolution is equipped with a Philips PW-1729 X-ray generator and a Philips PW1710/00 diffraction control unit with a PW-1775 35 port automatic sample changer. Instrument settings for all standards were as follows:

• Generator = 40 kV and 35 mA

• Tube anode = Cu

• Wavelength = 1.54056 Å (CuK_α)

• Intensity ratio = 0.5

• Focus = fine

• Irradiated length = 12 mm

• Divergence slit = automatic

• Receiving slit = 0.2 mm

• Step size = 0.02°2θ

• Count time per step = 1 s
  

• Scanning rate = 4°2θ/min

• Rate-meter constant = 0.2 s

• Spinner = off

• Monochronometer = on

• Scan = step

• Scanning from 2°2θ to 70°2θ

The software used for XRD data reduction was MacDiff (version 4.2.5), a shareware application for Macintosh computers that supports routine measurement of peak intensity and peak area.

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