Previous   |    Next

doi:10.2204/iodp.proc.303306.102.2006

Lithostratigraphy

This section outlines the procedures followed to document the basic sedimentology of the deposits recovered during IODP Expedition 303, including core description, X-ray diffraction (XRD) studies, color spectrophotometry, and smear slide description. Only general procedures are outlined, except where they depart significantly from Ocean Drilling Program (ODP) and IODP conventions.

Sediment classification

The sediments recovered during Expedition 303 were composed of biogenic and siliciclastic components. They were described using the classification scheme of Mazzullo et al. (1988). The biogenic component is composed of the skeletal debris of open-marine calcareous and siliceous microfauna (e.g., foraminifers and radiolarians, respectively) and microflora (e.g., calcareous nannofossils and diatoms, respectively) and associated organisms. The siliciclastic component is composed of mineral and rock fragments derived from igneous, sedimentary, and metamorphic rocks. The relative proportions of these two components are used to define the major classes of “granular” sediments in the scheme of Mazzullo et al. (1988).

The lithologic names assigned to these sediments consist of a principal name and modifiers based on composition and degree of lithification and/or texture as determined from visual description of the cores and from smear slide observations. The total calcium carbonate content of the sediments, determined on board (see “Sedimentary inorganic and organic carbon, nitrogen, and sulfur concentrations” in “Geochemistry”), also aided in classification. For a sediment that is a mixture of components, the principal name is preceded by major modifiers (in order of increasing abundance) that refer to components making up >25% of the sediment. Minor components that represent between 10% and 25% of the sediment follow the principal name (after “with”) in order of increasing abundance. For example, 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. Although these minor sedimentary components, ranging in abundance from 10% to 25%, are reflected in the sediment name in the description column as “with,” these components are not designated in the Graphic Lithology column of the barrel sheets.

These naming conventions follow the ODP sediment classification scheme (Mazzullo et al., 1988), with the exception that a separate “mixed sediment” category was not distinguished during Expedition 303 (Fig. F1). As a result, biogenic sediments are those that contain >50% biogenic grains and <50% siliciclastic grains, whereas siliciclastic sediments are those that contain >50% siliciclastic grains and <50% biogenic grains. During Expedition 303, neritic and chemical sediments were not encountered except as accessory components; therefore, these categories are not addressed below. Sediments containing >50% silt- and sand-sized volcanic grains were classified as ash layers. Size divisions for siliciclastic grains are those of Wentworth (1922) (Fig. F2), with 10 major textural categories defined on the basis of the relative proportions of sand, silt, and clay (Fig. F3); however, distinguishing between some of these categories is difficult (e.g., silty clay versus clayey silt) without accurate measurements of grain size abundances. The term “clay” is only used to describe particle size and is applied to both clay minerals and other siliciclastic material <4 µm in size. Size-textural qualifiers were not used for biogenic 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 on the dominant composition as described below:

• Sediments composed predominantly of calcareous, pelagic organisms (e.g., calcareous nannofossils and foraminifers): the lithification terms “ooze” and “chalk” reflect whether the sediment can be deformed with a finger (ooze) or can be scratched easily by a fingernail (chalk).

• Sediments composed predominantly of siliceous microfossils (diatoms, radiolarians, and siliceous sponge spicules): the lithification terms “ooze” and “radiolarite/spiculite/diatomite” reflect whether the sediment can be deformed with a finger (ooze) or cannot be easily deformed manually (radiolarite/spiculite/diatomite).

• Sediments composed predominantly of 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 (e.g., “clay”). For more consolidated material, the lithification suffix “-stone” is appended to the dominant size classification (e.g., “claystone”).

• Sediments composed of sand-sized volcaniclastic grains: if the sediment can be deformed easily with a finger, the interval is described as ash. For more consolidated material, the rock is called tuff.

Visual core description

Preparation for core description

The standard method of splitting a core by pulling a wire lengthwise through its center tends to smear the cut surface and obscure fine details of lithology and sedimentary structure. When necessary during Expedition 303, the archive halves of cores were gently scraped across, rather than along, the core section using a stainless steel or glass scraper to prepare the surface for unobscured sedimentologic examination and digital imaging. Scraping parallel to bedding with a freshly cleaned tool prevented cross-stratigraphic contamination.

Sediment barrel sheets

Core-description forms, or “barrel sheets,” provide a summary of the data obtained during shipboard analysis of each sediment core. Detailed observations of each section were initially recorded by hand on standard IODP visual core description (VCD) forms. Copies of original VCD forms are available from IODP upon request. This information was subsequently entered into AppleCORE software (version 9.4), which generates a simplified, annotated graphical description (barrel sheet) for each core (Fig. F4). Site, hole, and depth (in mbsf or mcd, if available) are given at the top of the barrel sheet, with the corresponding depths of core sections along the left margin. Columns on the barrel sheets include Graphic Lithology, Bioturbation, Structures, Accessory Components, Sediment Disturbance, Sample Types, Color, and Remarks. These columns are discussed in more detail below.

Graphic lithology

Lithologies of the core intervals recovered are represented on barrel sheets by graphic patterns in the column titled Graphic Lithology, using the symbols illustrated in Figure F5. A maximum of two different lithologies (for interbedded sediments) or three different components (for a uniform sediment) can be represented within the same core interval. Minor lithologies, present as thin interbeds within the major lithology, are shown by a dashed vertical line dividing the lithologies. Lithologic abundances are rounded to the nearest 10%; lithologies that constitute <10% of the core are generally not shown but are listed in the Description section. However, some distinctive minor lithologies, such as ash layers, are included graphically in the lithology column. Contact types (e.g., sharp, scoured, and gradational) are also shown in the Graphic Lithology column. Relative abundances of lithologies reported in this way are useful for general characterization of the sediment but do not constitute precise, quantitative observations.

Bioturbation

Five levels of bioturbation are recognized using a scheme similar to that of Droser and Bottjer (1986). Bioturbation intensity is classified as abundant (>75%), common (>50%–75%), moderate (10%–50%), rare (<10%), and absent (none). These levels are illustrated with graphic symbols in the Bioturbation column (Fig. F6).

Sedimentary structures

The locations and types of sedimentary structures visible on the prepared surfaces of the split cores are shown in the Structure column of the core description form. Symbols in this column indicate the locations of individual bedding features and any other sedimentary features, such as scours, ash layers, ripple laminations, and fining-upward or coarsening-upward intervals (Fig. F6).

Accessory components

Lithologic, diagenetic, and paleontologic accessories, such as nodules, sulfides, and shells, are indicated on the barrel sheets. The symbols used to designate these features found in Expedition 303 sediments are shown in Figure F6.

Sediment disturbance

Drilling-related sediment disturbance that persists over intervals of ≥20 cm is recorded in the Disturbance column using the symbols shown in Figure F6. The degree of drilling disturbance is described for soft and firm sediments using the following categories:

• 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 slurry, and other soft sediment stretching and/or compressional shearing structures attributed to coring/drilling. The particular type of deformation may also be noted (e.g., flow-in, gas expansion, etc.).

• Soupy: intervals are water saturated and have lost all aspects of original bedding.

Sample types

Sample material taken for shipboard sedimentologic and chemical analyses consisted of pore water whole rounds, micropaleontology samples, smear slides, and discrete samples for XRD. Typically, one to five smear slides were made per core, one pore water sample was taken at a designated interval, and a micropaleontology sample was obtained from the core catcher of most cores. XRD samples were taken only where needed to assess the lithologic components. Additional samples were selected to better characterize lithologic variability within a given interval. Tables summarizing relative abundance of sedimentary components from the smear slides were also generated.

Color

Color is determined qualitatively using Munsell Soil Color Charts (Munsell Color Company, Inc., 1994) and described immediately after cores are split to avoid color changes associated with drying and redox reactions. When portions of the split core surface required cleaning with a stainless steel or glass scraper, this was done prior to determining the color. Munsell color names are provided in the Color column on the barrel sheet, and the corresponding hue, value, and chroma data are provided with the Remarks.

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 and disturbances resulting from the coring process.

Smear slides

Smear slide samples were taken from the archive halves during core description. For each sample, a small amount of sediment was removed with a wooden toothpick, dispersed evenly in deionized water on a 1 inch × 3 inch glass slide, and dried on a hot plate at a low setting. A drop of mounting medium and a 1 inch × 1 inch cover glass was added, and the slide was placed in an ultraviolet light box for ~30 min. Once fixed, each slide was scanned at 100×–200× with a transmitted light petrographic microscope using an eyepiece micrometer to assess grain-size distributions in clay (<4 µm), silt (4–63 µm), and sand (>63 µm) fractions. The eyepiece micrometer was calibrated once for each magnification and combination of ocular and objective, using an inscribed stage micrometer.

Relative proportions of each grain size and type were estimated by microscopic examination. Note that smear slide analyses tend to underestimate the abundance of sand-sized and larger grains (e.g., foraminifers, radiolarians, and siliciclastic sand) because these are difficult to incorporate into the smear. Clay-sized biosilica, being transparent and isotropic, is also very difficult to quantify. Clay minerals, micrite, and nannofossils can also be difficult to distinguish at the very finest (less than ~4 µm) size range. After scanning for grain-size distribution, several fields were examined at 200×–500× for mineralogical and microfossil identification. Standard petrographic techniques were employed to identify the commonly occurring minerals and biogenic groups, as well as important accessory minerals and microfossils.

Smear slide analysis data tables are included in “Core descriptions.” These tables include information about the sample location, whether the sample represents a dominant (D) or a minor (M) lithology in the core, and the estimated percentages of sand, silt, and clay, together with all identified components.

Abundance of gravel-sized grains

One of the primary objectives of Expedition 303 was to collect cores that record the paleoclimatic history of the North Atlantic at millennial and longer timescales. Because of the geographic location and the geologic age of these sediments, ice-rafted debris (IRD) was expected to be an important part of that paleoclimatic record. The most easily identified IRD consists of outsized clasts in a fine-grained pelagic matrix and is often defined operationally as gravel-sized grains. During Expedition 303, the abundance of IRD was estimated by counting the number of gravel-sized grains (i.e., granule size and larger) in each 10 cm increment of the core. These counts were recorded and reported in tables that are available upon request from the IODP Data Librarian. The presence and composition of larger clasts (pebble size and larger) was noted.

Spectrophotometry (color reflectance)

Reflectance of visible light from the surface of split 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). Freshly split soft cores were covered with clear plastic wrap and placed on the AMST. Measurements were taken at 2.0 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 skip relatively small cracks, disturbed areas of core, or large clasts. Thus, AMST data may contain spurious measurements, which 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 analysis

Selected samples were taken for qualitative mineral analysis by using an XRD Philips model PW1729 X-ray diffractometer using Ni-filtered CuK_α radiation. Instrument conditions were as follows: 40 kV, 35 mA; goniometer scan from 2° to 70°2θ (air-dried samples); step size of 0.01°2θ; and count time of 1 s each. MacDiff software (version 4.1.1 PPC by Rainer Petschick) was used to display diffractograms, and identifications are based on multiple peak matches, using the mineral database provided with MacDiff.

Digital color imaging

Systematic, high-resolution, line-scan digital core images of the archive half of each core were obtained using the Geotek X-Y imaging system (Geoscan II). This DIS 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 width of the camera and core. The standard configuration for Geoscan II produces 300 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 for the acquisition of images with an ISA interface card for personal computers. After cores were visually described, they were placed in the DIS and scanned. A spacer holding a neutral gray color chip and a label identifying the section was placed at the base of each section and scanned along with each section. Output from the DIS includes a Windows bitmap (.BMP) file and a JPEG (.JPG) file for each section scanned. The bitmap file contains the original data. Additional postprocessing of data was done to achieve a medium-resolution JPEG image of each section and a composite JPEG image of each core, which is comparable to the traditional photographic image of each core. The JPEG image of each section was produced by an Adobe Photoshop batch job that opened the bitmap file, resampled the file to a width of 0.6 inches (~15 mm) at a resolution of 300 pixels/inch, and saved the result as a maximum-resolution JPEG. The DIS system was calibrated for black and white approximately every 12 h. No significant change in this calibration was observed during Expedition 303. A constant aperture setting of f/11 was used.

Top of page   |    Previous   |    Next