Lithostratigraphic procedures used during Leg 210 are described under the following headings: Sediment Classification, Visual Core Descriptions, Smear Slide and Thin Section Analyses, Whole-Core X-Ray CT Scans, Digital Imaging, Point Magnetic Susceptibility and Color Reflectance, and X-Ray Diffraction Analysis. In addition, the reader is referred to "Geochemistry" for an explanation of carbonate, major element, and trace element analysis of sedimentary rocks.
The Leg 210 lithologic classification (Fig. F2) 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 vs. limestone). The terms "conglomerate" and "breccia" are the principal names for consolidated gravels with well-rounded and angular clasts, respectively. A sedimentary rock composed of 65% gravel and 35% sand is termed a sandy conglomerate.
Many Leg 210 cores have borderline textures between claystone and mudstone, so different describers might use a different name for the same sediment. In such cases and to de-emphasize minor textural differences, the informal name "mudrock" is used widely in this volume as a synonym for undifferentiated claystone and mudstone. This informal name permits general features of all fine-grained facies to be described together but does not prevent the separate description, where appropriate, of particular features of the claystones or mudstones.
The rock name "grainstone" is utilized to describe a matrix-poor, silt- to sand-grade carbonate (Dunham, 1962). A significant number of the deepwater, coarse silt- to sand-grade turbidites cored at Site 1276 have a carbonate composition or a predominance of carbonate detritus (e.g., peloids, foraminifers, echinoderm fragments, ostracodes, and mollusk fragments) in a mixed carbonate-siliciclastic rock. To clearly distinguish these from siliciclastic siltstones and sandstones, the name grainstone is used for these rocks. This term is used even for sediment with a clay-sized matrix (either clay minerals or micrite) of several percent.
Coring during Leg 210 began at ~800 mbsf in lithified sediments, so sedimentary rock names are used throughout. Percentages of end-member grain components, as well as siliciclastic textures used to define lithologies reported on the visual core descriptions (VCDs), 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.
Many deposits encountered during Leg 210 are not end-member lithologies but instead grade from one lithology to another. A shorthand notation is used throughout this volume for such upward transitions within single gravity-flow deposits. This notation takes the form of an arrow (
), which indicates that one lithology gradually passes upward into another. Such transitions generally include sediments of intermediate grain size (e.g., sandstone mudstone usually means a gradual upward change through the various grades of sandstone, through silt-grade material, and eventually into mudstone through progressive fining).
Information from macroscopic and microscopic examination of each core section was recorded by hand on a primary description form ("barrel sheet"). This information was then condensed and entered into the AppleCORE program (version 8.1m) to generate simplified VCD forms (Fig. F3). Site, hole, and the depth interval in mbsf are given at the top of each VCD, with section intervals provided along the left margin. Copies of the original barrel sheets, which contain additional detailed core description, are available from the program data librarian by request. The columns depicted on the VCDs are discussed below.
The lithologic description on the right side of each VCD consists of five parts: (1) a heading that lists the sediment lithologies in the core; (2) a listing of the major lithologies in the core, together with adjectives to describe the color, state of bioturbation, and nature of bedding; (3) a listing of minor lithologies in the core with adjectives to describe the color, state of bioturbation, and nature of bedding; (4) an overall description of the core including details of facies present and their spatial relationships; and (5) a brief summary of structural features, where appropriate (see "Structural Geology").
Core ages are based on shipboard micropaleontological analyses, which are described in "Biostratigraphy".
A strip log of grain size is presented to the left of the Graphic Lithology column. This log is based on the estimated grain size of all terrigenous and biogenic components using the Wentworth (1922) scale. The reader should review the text description on each form to ascertain whether there are interbedded sedimentary rocks or simply mixed sedimentary rock types. Interbedded deposits have one or more vertical lines between the symbols for the separate rock types in the Graphic Lithology column (see below).
The key for rock types (or components in biogenic/terrigenous mixtures) and facies contacts is shown in Figure F4. Lithologic symbols for mixtures of biogenic and terrigenous particles are arranged in the Graphic Lithology column with, from left, terrigenous components from coarsest to finest, carbonate, and biogenic silica components. The exception to this rule is grainstone, which, although a carbonate, is plotted to the far left with sandstone. For interbedded sediments, a single vertical bar between patterns indicates thick bedding (30–100 cm), two vertical lines indicate medium bedding (10–30 cm), and three vertical lines indicate thin bedding (3–10 cm).
A common lithologic style observed in sediment cores during Leg 210 and conjugate-margin Legs 149 and 173 is upward-darkening "motifs," as much as several tens of centimeters thick, consisting of a relatively coarse grained and light-colored calcareous sediment at the base (e.g., calcareous sandy siltstone), overlain by fine-grained, light-colored calcareous claystone to marlstone, and capped by dark claystone or mudstone. The motifs are mostly interpreted as turbidites. Two-component, three-component, and four-component motifs are illustrated elsewhere in this volume (see Fig. F12 in the "Site 1276" chapter). Over intervals in which the motifs are too thin to be shown individually (in Cores 210-1276A-9R, 10R, 11R, 23R, and 24R), they are represented in the Graphic Lithology column by parallel columns scaled to reflect the proportion of each component lithology. Over the same interval, in the Sedimentary Structure column, symbols for "graded bedding" and "interbedding" (as very thin to thin or thin to medium alternations) are used to emphasize the interbedded nature of the deposits.
Bioturbation is shown by the darkness of a vertical bar to the right of the Graphic Lithology column (Fig. F5). The shades shown in this bar have the AppleCORE names that follow, corresponding to the numerical classification of Droser and Bottjer (1986):
The bar is left empty for structureless intervals that appear to be barren but might contain undetected burrows. The locations and types of sedimentary and tectonic structures, lithologic accessories, ichnofossils, body fossils, and diagenetic features are summarized in other columns to the right. Symbols for these features and components are presented in Figure F5.
Different types of drilling disturbance are recorded graphically on each VCD; symbols for the types and degrees of disturbance are shown in Figure F5. Drilling disturbance of lithified sediments is classified into four categories:
Over some intervals, no fractures were present at the time the descriptions were made—in such cases, the Drilling Disturbance column is left blank. It was noted while using the computed tomography (CT) scanner (see "Whole-Core X-Ray CT Scans") that some cores consisted of long pieces of unbroken rock, whereas after splitting with the saw they were slightly fractured throughout. Clearly, fractures seen in split cores do not all result from drilling and may in part be produced during other stages of core handling.
Only the positions of smear slides (SS), micropaleontology samples (PAL), microbiology samples (MBIO), and whole-core samples (WHC) are indicated in the Sample column of the VCDs. The locations of other shipboard samples can be found by searching the Janus database on the Integrated Ocean Drilling Program Web site. The appropriate sample codes are:
Color was determined by comparison with the Rock Color Chart published by the Geological Society of America in 1984. Color codes are presented in the "Colour" column of the VCDs.
Smear slides could be prepared from the moderately consolidated sedimentary rocks and even from some of the fully lithified ones. 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 x 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 x 75 mm glass microscope slide, after which the coverslip was transferred onto the slide. The slide and coverslip were then 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 slide. Some scientific party members 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 proportions and presence of biogenic and mineral components. Thin sections were described using the same petrographic microscope. Digital photographs were acquired using a Nikon CoolPIX 990 camera with an adapter tube.
Smear slide and thin section data are included in data tables in "Core Descriptions." In these tables, components are assigned to one of the following categories:
The reader should be aware that quartz and feldspar often cannot be distinguished in smear slides. X-ray Diffraction (XRD) analysis of Leg 210 samples shows considerably higher feldspar content than was estimated from smear slides.
An X-ray CT scanner provides rapid acquisition of X-ray images without destruction of samples (e.g., Soh, 1997; Shipboard Scientific Party, 2002). An X-ray beam passes through the sample material, in this case a core and its liner, and is attenuated by interaction with the various constituents in the sample. First, a set of two-dimensional (2-D) X-ray images is acquired at different orientations relative to the core axis; these show integrated densities across the entire thickness of the core. The 2-D image set is then stacked to provide a three-dimensional (3-D) representation of the density variation in the core. This can be sliced in any orientation for later study. These 2-D slices through a 3-D X-ray data cube lack the blurring found in traditional 2-D X-radiographs and are caused by the thickness of the sample. For slices through the data cube, the density distribution is integrated over a thickness of 1 mm or less and it therefore gives a spatially precise map of density variation in the core. The processed 3-D CT images can be viewed using free software, IMAGEJ, available from the U.S. National Institutes of Health. During Leg 210, only vertical and horizontal slices were examined.
The same portable X-ray CT system used during Leg 204 was installed on the JOIDES Resolution for Leg 210. The unit was manufactured by Lawrence Berkeley National Laboratory and is described in detail by Freifeld et al. (2003). During scanning, the core is rotated around its vertical axis. The gantry holding the X-ray source and detector is raised and lowered by a belt-driven actuator to image selected regions of the core. The X-ray source has a tungsten target and a 250-µm beryllium window, delivering as much as 130 kV at 65 W. It has a variable focal-spot size that increases from 5 µm at 4 W to 100 µm at 65 W. A 15-cm image intensifier with a cesium iodide phosphor input screen is coupled to a digital camera for image capture (a Sony XC-75 with a resolution of 768 pixels x 494 pixels and a signal-to-noise ratio of 56 dB). Ten frames of the same image are acquired and averaged to reduce camera noise. A high-resolution monochrome monitor provides real-time viewing of the X-ray images.
During routine core handling, as time permitted, a variable proportion of the sections from recovered cores were viewed in a "quick-scan" mode by rapidly raising the gantry through the 150-cm section without rotation. This streamed 2-D X-ray images to a video monitor. Interesting 9-cm-long intervals that showed sufficient density contrast were then selected for detailed scanning. In these intervals, high-resolution images were obtained and stored digitally for subsequent 3-D reconstruction. The resolution of individual images is ~0.1 mm; 180 separate scans were collected during each rotation about the vertical axis of the section (i.e., a scan for each 2° rotational step).
Many of the sets of high-resolution images could not be used for 3-D reconstruction because the core pieces wobbled in the plastic liner during 360° rotation of the gantry. This was a problem with many of the rotary core barrel (RCB) cores because they were undersized.
Digital processing of the high-resolution images was performed with a National Instruments PCI–409 10-bit frame grabber installed on a personal computer. Image reconstruction software, Imgrec (developed at Lawrence Livermore National Laboratory), was employed to perform convolution backprojection and Feldkamp reconstructions of the acquired cone-beam radiographs. The Feldkamp algorithm corrects for the divergent cone beam angles and can provide an accurate reconstruction of 180 images of a 10-cm section of core in 20 min using a 2.4-GHz Pentium IV processor.
All archive halves were scanned using a Geotek 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 at an 8-bit dynamic range. The standard DIS configuration acquires 300 dpi on an 8-cm-wide core. Synchronization and track control is better than 0.02 mm. A framestore card contains 48 MB of RAM for image acquisition. The camera aperture was set to maximize contrast for 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 TIFF (no compression) and SID (compressed) versions. Using the Geotek Image Tools utilities, the SID files were resampled to produce a JPEG file with a resolution of ~300 dpi. The JPEG files are viewable using 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 was calibrated for black and white whenever deemed necessary by ODP technical staff. The individual SID images were routinely viewed using MrSID Standalone Viewer (version 2.0). This capability was essential for reviewing previous cores and for preparation of draft figures for site reports.
Archive-half sections were measured at 2-cm intervals on the AMST with a Bartington MS2 magnetic susceptibility meter at a sensitivity setting of 0.1 instrument units.
Color was measured on the split-core surface of the archive half of selected cores using diffuse-reflected spectrophotometry, available on the AMST. Light reflected from the material was collected in an integration sphere normalized to the source light of the reflectance and calibrated with the measurement of a pure white standard (100% reflection) and a black box (0% reflection) over the entire wavelength spectrum of visible light. Shipboard reflectance was measured using an automated Minolta photospectrometer (CM-2002), which measures the spectral reflectance of surfaces with a diameter >8 mm. To ensure accuracy, the CM-2002 uses a double-beam feedback system, monitoring the illumination on the specimen at the time of measurement and automatically compensating for any changes in the intensity of spectral distribution of the light.
Color is reported using the L*a*b* system. This can be visualized as a cylindrical coordinate system in which the axis of the cylinder is the lightness component L*, ranging from 0% to 100%, and the radii are the chromaticity components a* and b*. Component a* is the green (negative) to red (positive) axis, and component b* is the blue (negative) to yellow (positive) axis.
Selected samples were taken for qualitative mineral analysis by XRD using a Philips diffractometer with CuK
radiation generated at 40 kV and 35 mA and with a focusing graphite monochromator. The settings were as follows:
.
/min.
Bulk samples were freeze-dried, ground with an agate mortar and pestle, and tightly packed in sample holders. The packing, together with the ship's movement, probably imparted some orientation to the mineral powder. These samples were scanned from 2° to 70°2. MacDiff software (version 4.2.5 PPC, freeware distributed by Rainer Petschick) was used to display diffractograms and to identify the minerals. Most diffractograms were corrected to match the main peaks of quartz and calcite at 3.343 and 3.035 Å, respectively. Identifications are based on multiple peak matches with the mineral database provided with the MacDiff software. Relative abundances reported in the site chapters and data tables are useful for general characterization of the sediments, but they are not quantitative concentration data.
