LITHOSTRATIGRAPHY

Sediment Barrel Sheets

Data obtained during shipboard analysis of each sediment core are summarized on core description forms termed "barrel sheets." To generate barrel sheets, detailed observations of each core or core section were first recorded on standard ODP visual core description (VCD) forms. Copies of these original, handwritten VCD forms are available from ODP upon request. Information from the VCD forms for each core was then compiled and entered into AppleCORE (version 8.1m). AppleCORE was used to draft a simplified annotated graphical representation (i.e., the barrel sheet) of each core (Fig. F1). Barrel sheets appear in the Core Descriptions for each site (see the "Core Descriptions" contents list), and each barrel sheet contains a link to the corresponding core photograph. The header of each barrel sheet lists the site, hole, core, and interval cored in mbsf. Depth (in mbsf) is plotted relative to the core sections along the left edge of the barrel sheet; where recovery is <100%, the recovered interval is placed at the top of the cored interval. Other columns on the barrel sheet are Lightness, Lithology, Bioturbation, Disturbance, Samples, and Remarks. Conventions used during Leg 207 in codifying observations and depicting measurements are discussed in more detail below.

Graphic Lithology

Major lithologies recovered in each core are represented on barrel sheets by graphic patterns (Fig. F2) in the Graphic Lithology column. Components that represent 25% or more of the total sediment are depicted. For intervals where different lithologies are interbedded at a fine scale, the variability is shown schematically. For intervals containing a homogeneous mixture of two or more lithologies (e.g., clayey nannofossil ooze), the appropriate patterns are all shown with the width of each pattern in the column approximating the relative abundance of that component. Note that these abundance estimates are based on qualitative observations of core samples and smear slides by multiple observers. They are useful for determining the general character of the sediment and for recognizing stratigraphic trends, but they are not appropriate for quantitative analyses.

Sedimentary Structures and Minor or Accessory Phases

Sedimentary structures are considered discrete features formed by natural biological or physical processes (i.e., they do not include drilling disturbance). They are represented on the barrel sheet by symbols placed in the Graphic Lithology column at the depth level at which they were observed. Structures noted include the nature of bedding contacts, types of trace fossils, presence of laminae, macro- and microfaults, and structures associated with soft sediment deformation (Fig. F3).

Symbols are also used to denote accessory lithologies, authigenic minerals, concretions, fossils, and coring-induced sediment disturbance (Fig. F3). As above, these symbols are positioned near 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 extent of its presence.

Bioturbation

Five levels of bioturbation are recognized using a scheme similar to that of Droser and Bottjer (1986). Bioturbation is classified as the following:

  1. Pervasive: >75% of interval is burrowed and/or primary bedding contacts are destroyed.
  2. Heavy: >50%–75% of interval is burrowed and/or primary bedding contacts are rare.
  3. Moderate: 10%–50% of interval is burrowed and/or primary bedding contacts are common.
  4. Slight: <10% of interval is burrowed and/or primary bedding contacts are rarely cut by discrete traces.
  5. Absent: no traces observed and/or all primary bedding contacts are preserved.

These levels are illustrated as a grayscale bar in the Bioturbation column (Fig. F3).

Color

During visual core description, sediment color was estimated qualitatively using Munsell Soil Color Charts (Munsell Color Company, 1994). To minimize color changes associated with drying and redox reactions, observations were made soon after each core was split. Munsell color names and a summary of color variation are provided with the lithologic summary and general lithologic description in the Remarks column of each barrel sheet. Quantitative color data were collected using color reflectance spectrophotometry (see "Color Reflectance Spectrophotometry").

Sample Types

The position of samples taken for shipboard sedimentological, paleontological, and chemical analyses are shown in the Sample column. Sample types include interstitial water whole rounds (IW), micropaleontology samples (PAL), smear slides (SS), thin section billets (THS), inorganic carbon (CAR), discrete samples for identification of mineral components using X-ray diffraction (XRD), and whole rounds for either physical properties (WRP) or microbiology (WRB). Depending on lithologic variability, typically one to six smear slides were made per core. In addition, pore water samples were taken at designated intervals and a micropaleontology sample was usually obtained from the core catcher. Inorganic carbon and XRD samples were taken where needed to assess the lithologic composition and components. Additional samples were selected to better characterize lithologic variability in a given interval. Tables summarizing relative abundance of sedimentary components observed in the smear slides and thin sections were generated using a spreadsheet program (Sliders) and are presented with the barrel sheets (see the "Core Descriptions" contents list).

Remarks

The written description for each core contains a brief overview of both major and minor lithologies present and notable features such as sedimentary structures, fossils, and disturbances resulting from the coring process.

Drilling Disturbance

The extent and style of disturbance introduced during drilling and recovery in shown is the Drilling Disturbance column (Fig. F1), and the symbols used are shown in Figure F3.

Sediment Classification

Naming conventions generally follow the ODP sediment classification scheme (Mazzullo et al., 1988), but we did not use the "mixed sediment" category. Lithologic names consist of a principal name, which is based on composition and degree of lithification and/or texture as determined from visual description and smear slide observation. For a mixture of components, the principal name is preceded by major modifiers (in order of increasing abundance) that refer to components that make up >25% 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. Using this scheme, an unconsolidated sediment containing 30% nannofossils, 25% clay minerals, 20% foraminifers, 15% quartz silt, and 10% manganese nodules is a "clayey nannofossil ooze with manganese nodules, quartz silt, and foraminifers." Note that minor sedimentary components (10%–25%) are included in the sediment name, but these components do not appear on the column of the barrel sheet in which lithology is graphically depicted. During Leg 207, neritic and chemical sediments were not encountered except as accessory minerals; therefore, these categories are not discussed further.

Granular sediments were subdivided on the basis of composition and abundance of different grain types. The values were estimated from visual examination of the core, smear slides, and thin sections and were augmented by shipboard measurements of carbonate content and shipboard XRD analyses (see "X-Ray Diffraction"). In volcaniclastic sediments, the term ash (tuff if lithified) is used instead of sand. Size divisions for grains are those of Wentworth (1922) (Fig. F4), but size-textural qualifiers were not used in generating pelagic sediment names (e.g., the name nannofossil clay denotes a sediment in which the dominant component is detrital clay rather than clay-sized nannofossils).

Terms that describe lithification vary depending upon the dominant composition as described below:

  1. Sediments 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.
  2. Sediments derived predominantly from siliceous microfossils (diatoms, radiolarians, and siliceous sponge spicules):
    Ooze: sediment can be deformed with a finger;
    Radiolarite/spiculate/diatomite: sediment cannot be easily deformed manually;
    Chert: sediment displays a glassy luster.
    Use of the term porcellanite follows the definition of Keene (1975) and describes a siliceous limestone/claystone that has a dull luster and is less hard and compact than chert. 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.
  3. Sediments 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 vs. claystone).
  4. Sediments composed of sand-sized volcaniclastic grains:
    Ash: sediment can be deformed easily with a finger;
    Tuff: more consolidated material.

Color Reflectance Spectrophotometry

In addition to visual estimates of color, reflectance of visible light from soft-sediment cores was routinely measured using a Minolta spectrophotometer (model CM-2002) mounted on the AMST. The AMST measures the archive half of each core section and provides a high-resolution stratigraphic record of color variations for visible wavelengths (400–700 nm). Lightness (L*) values are plotted on the barrel sheets; these values as well as all other color measurements are in the Janus database.

To measure reflectance, freshly split cores were covered with clear plastic wrap and placed on the AMST. Measurements were typically taken at 2.5-cm spacing. The AMST skips empty intervals and intervals where the core surface is well below the level of the core liner but does not recognize relatively small cracks or disturbed areas of core. Thus, AMST data may contain spurious measurements that should be edited out of the data set. Each measurement recorded consists of 31 separate determinations of color 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) and Balsam and Damuth (2000).

X-Ray Diffraction

Selected samples were taken for qualitative mineral analysis using a Philips PW-1729 X-ray diffractometer with a CuK source (40 kV and 35 mA). The goniometer scan was performed from 2 to 702 for the air-dried samples analyzed during Leg 207. Scan speed was 1.2/min (step = 0.01 and count time = 0.5 s).

Some samples were decalcified using 10% acetic acid then washed repeatedly with demineralized water, centrifuged, and decanted. The carbonate-free fraction was deflocculated with a 1% Calgon (sodium hexametaphosphate) solution and homogenized in a sonic dismembrator for 1 min. The interactive software package MacDiff (version 4.1.1) (Petschick, 2000) was used to display diffractograms, and identifications are based on multiple peak matches using the mineral database provided with MacDiff. Diffractograms were peak corrected to match the calcite peak at 3.035 . In the absence of calcite, no peak correction was applied.

Digital Color Imaging System

Systematic high-resolution line-scan digital core images of the archive half of each core were obtained using the Geotek X-Y digital imaging system (Geoscan II). This system collects digital images with three line-scan charge-coupled device arrays (1024 pixels each) behind an interference filter to create three channels (red, green, and blue). The image resolution is dependent on the height of the camera and width of the core. The standard configuration for the Geoscan II produces 300 dots per inch (dpi) on an 8-cm-wide core, with a zoom capability up to 1200 dpi on a 2-cm-wide core. Synchronization and track control are better than 0.02 mm. The dynamic range is 8 bits for all three channels. The Framestore card has 48 MB of onboard random access memory (RAM) for the acquisition of images with an ISA interface card for personal computers. After cores were visually described, they were placed in the digital imaging system and scanned. A spacer holding a neutral gray color chip and an identifying label was scanned with each section. Output from the digital imaging system includes a Windows bitmap (.BMP) file and a Mr.Sid (.SID) file for each section scanned. The bitmap file contains the original data with no compressional algorithms applied, whereas the Mr.Sid files contain extensive compressional algorithms. The digital imaging system was calibrated for black and white approximately every 12 hr.

Because sediments and sedimentary rocks ranging from black to white were recovered during Leg 207, sometimes it was necessary to adjust the aperture of the imaging system to prevent resolution loss resulting from over- or underexposure. To minimize artifacts in composite images related to changes in the aperture, we changed the f/stop only when light-colored intervals oversaturated the sensors or when there was a change to markedly darker lithologies that extended over several cores. In exceptional cases where very light and very dark lithologies were present in the same core section, the core section was scanned twice—once each with an appropriate f/stop for the light and dark intervals, respectively.

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