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doi:10.2204/iodp.proc.311.102.2006

Lithostratigraphy

The techniques and procedures used to describe, analyze, and name the lithologies recovered during Expedition 311 are described below. These include VCDs, smear slide and thin section descriptions, XRD analyses, color spectrophotometry, and high-resolution digital color imaging. Any significant deviations from the procedures outlined in this section are discussed in the individual site chapters.

Sediment classification

The naming conventions adopted during Expedition 311 follow a slightly modified version of the ODP sediment classification scheme of Mazzullo et al. (1988). We do not distinguish mixed sediments but only siliciclastic and/or volcaniclastic (>50% components), biogenic (>50% components), and diagenetic (>50% components) sediments. Principal names were assigned to sediments based on composition, texture, and degree of lithification as determined primarily from visual description and smear slide analyses. Modifiers to the principal name were determined based on both the abundance and type of the nonprincipal component or components (e.g., siliciclastic or biogenic). Major modifiers, those components that compose >25% of the sediment, precede the principal name and are listed in order of increasing abundance. Genetic terms, such as pelagic, neritic, and debris flow, were not used in classifying the sediments and are used only in geologic interpretations of the sedimentary sequence. Minor modifiers, those components that compose 10%–25% of the sediment, follow the principal name; they are preceded by the term "with" and are listed in order of increasing abundance.

Siliciclastic sediments

For sediments and rocks composed of >50% siliciclastic components, the principal name was determined by the texture of the grains. Textural names were derived from the Udden-Wentworth grain-size scale (Wentworth, 1922) (Fig. F2). In this classification scheme, the term "clay" is independent of mineralogy and refers to all siliciclastic grains <3.9 Ám in size, regardless of composition. The relative proportion of different grain sizes was determined by visual percentage estimation using the comparison chart of Terry and Chilingar (1955). Once the relative proportions were determined, a modified Shepard (1954) classification scheme was used to assign the principal name (Fig. F3). Clay, silt, and sand are the principal names in the Shepard diagram. If any component exceeds 25% of the total siliciclastic grains, it becomes a modifier to the principal name. For example, sediment composed of 10% clay and 90% silt is simply a silt, whereas sediment composed of 30% clay and 70% silt is a clayey silt.

The lithologic classification of sediments was refined by adding major and minor modifiers. 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%) and minor (10%–25%) proportions.

Biogenic sediments

Unlike siliciclastic sediments, biogenic sediments, defined as containing >50% biogenic components, are not described based on texture. Rather, the principal name for all biogenic sediments is ooze. Thus, a sediment with 65% sand-sized foraminifers and 35% siliciclastic clay is called clayey foraminifer ooze.

Authigenic carbonates

During Expedition 311, unlithified, partly lithified, and lithified authigenic carbonates (e.g., aragonite, calcite, dolomite, siderite, and rhodochrosite) were encountered as nodules or irregular precipitates. Different symbols were given for unlithified carbonate cements and for lithified/partly lithified carbonates (Fig. F4). These symbols appear in the Diagenesis column of the VCD forms ("barrel sheets") (Fig. F5). In cases where it was possible to clearly identify the carbonate mineralogy, it was given in the VCD form. Carbonates are mostly composed of tiny calcite and/or dolomite crystals (Fig. F6). They are classified according to their mineralogy, grain size (<4 Ám: micritic; 4–63 Ám: microcrystalline), and degree of lithification (unlithified, partly lithified, or lithified carbonate). The principal name is followed (preceded by "with") by any modifier (>10% component). For example, a lithified carbonate composed of crystals <4 Ám long with 10% quartz would be named micritic lithified carbonate with quartz.

Firmness

Calcareous sediments and rocks are divided into three classes of firmness:

  1. Unlithified = soft sediments that readily deform under the pressure of a fingernail or spatula.
  2. Partly lithified = firm but friable sediments that can be scratched with a fingernail or the edge of a spatula.
  3. Lithified = hard, nonfriable cemented rocks that are difficult or impossible to scratch with a fingernail or the edge of a spatula.

Visual core descriptions

Detailed sedimentologic observations and descriptions were recorded manually for each core section on VCD forms. A wide variety of features that characterize the sediments were recorded, including lithology, bioturbation, sedimentary structure, fossils, core disturbance, diagenetic precipitates, samples, and color. Compositional data were obtained from smear slides. The color (hue and chroma) of the sediments was determined by both color spectrophotometry and by visual comparison with the Munsell soil color chart (Munsell Color Company, 1975). This information was synthesized for each core in AppleCORE (v9.4a), which generates a one-page graphic description (barrel sheet) of each core (Fig. F5). Barrel sheet symbols used during Expedition 311 are described in Figure F4. For more detailed information on sedimentary features, VCD forms are available from IODP upon request.

Of particular interest during Expedition 311 were the visual indications of disruption to the sediment caused by the dissociation of gas hydrate in the recovered sediment noted in the Disturbance column. Massive forms of gas hydrate were removed on the catwalk prior to core description and sampled intervals were noted in the barrel sheets. The two primary textures identified as resulting from the dissociation of gas hydrate are soupy and mousselike. Soupy sediments are watery, homogeneous, and fluidized (Fig. F7). These sediments are often associated with centimeter-scale void spaces in the core because they are able to flow from their original position during core recovery and therefore retain no original sedimentary structures. Sediments with mousselike texture contain small millimeter-scale voids and obscure primary sedimentary structures (Fig. F8). These sediments are often wet, soft, and deform plastically under slight pressure from one finger.

Mousselike and soupy textures related to the dissociation of gas hydrate not sampled prior to description were noted on the barrel sheets. Remarks made on barrel sheets for each core describe any additional potential indications of gas hydrate near the sampled intervals, including the presence of dry, flaky sediment that may have been dewatered by the formation of gas hydrate nearby.

Lithology and grain size

The lithology of the described sediments is represented in the Graphic Lithology column of the barrel sheets using the symbols shown in Figure F4. A maximum of three different lithologies (for interbedded sediments) or three different components (for mixed sediments) can be represented within the same core interval. Intervals >1 cm in thickness can be portrayed accurately in the lithology column. Percentages are rounded to the nearest 10%, and lithologies that constitute <10% of the core are generally not shown but are listed in the Description column.

Bioturbation

Visible bioturbation was classified into four intensity levels based on the degree of disturbance of the physical sedimentary structures:

  1. Absent = no bioturbation; all sedimentary structures are preserved.
  2. Rare = isolated trace fossils; up to 10% of sedimentary structures are disrupted.
  3. Moderate = ~10%–40% disrupted sedimentary structures; burrows are generally isolated but may overlap locally.
  4. Abundant = bedding completely disturbed; burrows are still intact in places.

These categories are based on the ichnofossil indexes of Droser and Bottjer (1986) and are illustrated with graphic symbols in the Bioturbation column on the barrel sheets. Visual recognition of bioturbation was often limited in homogeneous sediments, particularly in hemipelagic clay zones without sulfide material.

Sedimentary structures

Each type of sedimentary structure and its exact location is displayed in the Structure column of the barrel sheet. Symbols used to note the wide variety of sedimentary structures encountered throughout Expedition 311 are listed in the barrel sheet legend (Fig. F4). Some of the more common structures observed were parallel bedding, fining-upward sequences, and the mottled appearance of sulfide-rich layers.

Fossils

The presence of macroscopic fossils (including aggregates of sponge spicules, large foraminifers (~1 mm), bivalve shell fragments, preserved whole bivalve shells, and gastropods) is displayed in the Fossils column on the barrel sheets.

Sediment disturbance

Drilling-related sediment disturbance is recorded in the Disturbance column. Separate terms describe the degree of drilling disturbance in soft and firm sediments:

  • Slightly disturbed = bedding contacts are slightly deformed.
  • Moderately disturbed = bedding contacts have undergone extreme bowing.
  • Highly disturbed = bedding is completely deformed as flow-in, coring/drilling slough, and other soft sediment stretching and/or compressional shearing structures attributed to coring/drilling.

Further coring/drilling-related sediment disturbance observed includes

  • Drilling "biscuits" (drilling slurry surrounding an intact or slightly fractured dense mass of sediment) and
  • Gas-expansion cracks (cracking and pushing apart of sediments resulting from the expansion of gases during core retrieval). This process produces many, mostly centimeter-scale voids.

Gas hydrate–related sediment disturbance is recorded in the Disturbance column. Separate terms describe the degree of gas hydrate–related sediment disturbance:

  • Mousselike = texture is soft and deforms plastically under slight pressure. Some primary sedimentary structure is preserved.
  • Soupy = intervals are water saturated and have lost all primary sedimentary structures.

Cores recovered from gas- and gas hydrate–bearing sediments are often disturbed by gas expansion and fracturing. In cases where it was possible to distinguish between disturbance of the core resulting from drilling and disturbance resulting from gas expansion, notes were made in the Comments column of the barrel sheets listing the depths at which gas fracturing was observed.

Diagenesis

The relative positions of features that are related to diagenesis are displayed in the Diagenesis column on the barrel sheets. These are mineral precipitates (e.g., iron sulfide and carbonates).

Samples

The position of whole-round samples, as well as smear slides and samples taken to support and verify the observations of the smear slide and thin section analyses, are indicated in the Sample column on the barrel sheets. The abbreviations are as follows:

  • SS = smear slide.
  • THS = thin section.
  • XRD = X-ray diffraction.
  • IW = interstitial water.
  • MB = microbiology.
  • HYD = gas hydrate.
  • HS = headspace gas (only used when complete whole-round sample was taken).
  • PAL = micropaleontology.
  • OG = organic geochemistry.
  • WRP = whole-round physical property.

Analysis of smear slides and thin sections

Smear slides were prepared from the archive halves of the cores. With a toothpick, a small amount of sediment was taken and put on a 25 mm x 75 mm glass slide, homogenized, and dispersed over the slide with a drop of deionized water. The sample was then dried on a hot plate at ~80°C. A drop of Norland optical adhesive and a 22 mm x 22 mm cover glass were added. The smear slide was fixed in an ultraviolet light box. With a transmitted light petrographic microscope, both the grain size and abundance of dominant components in a sample were determined. Abundance was estimated with the help of a comparison chart for visual percentage estimation (after Terry and Chilingar, 1955). Note that smear slide analyses tend to underestimate the amount of sand-sized and larger grains because these grains are difficult to incorporate into the slide. Table T1 is an example of data obtained from smear slide analyses and was generated using a spreadsheet. This table includes information about the location of samples, their grain-size distribution, and whether the sample represents the dominant (D) or the minor (M) lithology in the core. Additionally, it provides estimates of the major mineralogical and biological components from the examination of each smear slide. The presence of authigenic minerals, such as iron sulfides or carbonates, as well as the presence of rare trace minerals, was noted in the Comments column. The mineralogy of the major smear slide components was also validated by XRD analyses, and the relative proportion of carbonate and noncarbonate material was validated by chemical analysis of the sediments (see "Organic geochemistry").

Thin sections were taken from several authigenic carbonate precipitates. Tables summarizing thin section data, such as grain size and relative abundance of sedimentary components, were also generated using spreadsheets and are available in "Core descriptions." A Zeiss Axioplan microscope equipped with a digital camera was used to obtain images of the smear slides and thin sections on board. Digital photomicrographs were obtained and stored as TIFF files. Thin section results complement the VCDs.

X-ray diffraction analyses

XRD analyses supported and verified smear slide and thin section descriptions. Each sample was freeze-dried, ground, and mounted with a random orientation into an aluminum sample holder. These measurements were made on a Philips PW-1729 X-ray diffractometer with a CuK source (40 kV and 35 mA) and Ni filter. Peak intensities were converted to values appropriate for a fixed slit width. The goniometer scan was performed from 2░ to 70░2 at a scan rate of 1.2░/min (step = 0.01░, count time = 0.5 s). Diffractograms were peak-corrected to match the (100) quartz peak at 3.343 ┼. Common minerals were identified based on their peak position and relative intensities in the diffractogram using MacDiff (v4.1.1) (R. Petschick; www.geologie.
uni-frankfurt.de/Staff/Homepages/Petschick/RainerE.html
).

Color reflectance spectrophotometry

In addition to visual estimates of color, reflectance of visible light from soft sediment cores was 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 record of downcore color variations for the visible wavelengths (400–700 nm). Freshly split cores were covered with clear plastic wrap and placed on the AMST. Measurements were taken at 5.0 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, to the extent possible, be edited out of the data set before use. Each measurement recorded consists of 31 separate determinations of reflectance in 10 nm wide spectral bands from 400 to 700 nm. Additional detailed information about the measurement and interpretation of spectral data with the Minolta spectrophotometer can be found in Balsam et al. (1997, 1998) and Balsam and Damuth (2000).

Digital color imaging

All core sections were imaged using the GeoTek X-Y DIS immediately after being split and scraped. We found it particularly useful to scrape the cores immediately prior to imaging to capture the ephemeral nature of some sedimentary features, particularly sulfide precipitates, which become oxidized within minutes of core splitting. The effect of oxidation can be seen in Figure F16 in the "Site U1328" chapter. It is worth noting that this fresh scraping did not occur prior to the regular photo imaging, which can take place between 30 and 60 min after scraping. Consequently, some of the sedimentary details may be lost using the more conventional archive core–table photographs. For this reason, cores were scraped before close-up photographs were taken. Some problems were encountered with occasional system crashes, which were probably a result of automated network uploading of files during image transfer from the camera hardware to the local computer.

In the DIS core section dialogue box, we set the section subbottom depth at zero for Section 1 and let it automatically increment down the remaining sections for each core. All images were acquired at a crosscore and downcore resolution of 100 pixels/cm. At the beginning of Expedition 311 coring operations, the aperture was fixed at a value that would image most cores without the need for further adjustment. It was, therefore, set to f/6.7 for Sites U1327 and U1329, which maximized the dynamic range for most of the core sections. At the beginning of Hole U1325B, the aperture was changed to f/5.6. Care was taken to ensure that the system was correctly calibrated using the "white tile" procedure and that the camera position was correctly set up. Output from the DIS corresponds to an uncompressed TIFF file (available upon request) for each scanned section. Additional postprocessing of the color imagery was done to achieve a medium-resolution JPEG image of each section.