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doi:10.2204/iodp.proc.327.102.2011

Lithostratigraphy

Note: This section was contributed by Sarah-Jane Jackett (Integrated Ocean Drilling Program, Texas A&M University, 1000 Discovery Drive, College Station TX 77845, USA).

The lithostratigraphic procedures used during Expedition 327, including sediment classification, visual core description, smear slide preparation and description, and XRD, are outlined below.

Core preparation

Prior to description, the quality of the split surface of the archive half of each core was assessed and, where necessary (e.g., surface was smeared or uneven), scraped lightly with a glass slide. This improved the visibility of sedimentary structures and fabric.

Visual core description

Sediment components and percentages in the core were determined using a hand lens, binocular microscope, and smear slide examination. Information from macroscopic and microscopic examinations of each core section was entered into the DESClogik program. Before core description, a customized template was created for recording lithology, drilling disturbance, and bioturbation. A second template containing columns for texture and relative abundance of biogenic/mineralogic components was configured specifically for recording smear slide data. DESClogik includes a graphic display mode for core data (e.g., digital images of section halves and measurement data) that can be used to augment core description. The data entered in DESClogik were uploaded into the LIMS database. The schemes used for sediment description are detailed below.

Lithologic classification

The lithologic classification scheme used during Expedition 327, modified after Shipboard Scientific Party (2004), is based on three end-member grain components: biogenic silica, carbonate, and terrigenous or volcanic grains, along with alternative modifiers determined from smear slides. This scheme is further divided according to the grain size of the terrigenous component (i.e., the relative proportions of sand, silt, and clay) (Wentworth, 1922) (Fig. F6). Adjectives such as “silty” and “clayey” are used to differentiate mixtures. Mixtures are divided on the basis of whether the silt content is greater than or less than 50% (silty clay mud or clayey silt, respectively). Sand is divided into fine sand and medium sand for description purposes.

Mixtures of terrigenous, siliceous, and calcareous sediments are named according to the relative proportion of the three components (Fig. F6). Sediment names also indicate the degree of sediment induration (e.g., clay versus claystone).

Color

Sediment color was determined qualitatively for core intervals using Munsell soil color charts (Munsell Color Company, Inc., 2000).

Sedimentary structures and lithologic accessories

Sedimentary structures, accessories, and other primary and secondary (diagenetic) features are noted in the core descriptions. Laminae are described as <1 cm thick. For turbidite sequences in which three lithologies are closely interbedded (i.e., the individual beds are <12 cm thick and alternate between the three lithologies), two interbedded lithology names are used (Fig. F6): interbedded fine sand-silt-clay and interbedded medium sand-silt-clay. When beds are scattered throughout a different lithology (e.g., beds of silt several centimeters to tens of centimeters thick within a clay bed), they are logged individually, and their associated thicknesses and textures are entered into the database using the DESClogik application.

Lithologic accessories noted include the presence of foraminifers, fossilized wood, glass, and mottles. Bioturbation is noted as either moderate or strong (Fig. F7).

Coring disturbance

The type and degree of coring disturbance are indicated according to the terminology defined in Figure F7.

Smear slide analysis

Toothpick samples were taken at select intervals in the core and used to create smear slides according to the method outlined in Mazzullo et al. (1988). One or more smear slide samples were collected from the main or dominant (D) lithology from the archive half of each core. Additional samples were collected from minor (M) lithologies or other areas of interest (e.g., laminations, mottles, etc.). Smear slides were viewed with a transmitted-light petrographic microscope, and the percentages of different mineralogic, biogenic, and authigenic components were estimated along with the proportions of sand, silt, and clay (terrigenous only). These estimations were recorded on smear slide sample sheets and entered into the database. This technique is limited in that sand grains are underemphasized, as are large calcareous components (shells and shell fragments), and the determination of percentages is subjective and varies slightly among different practitioners. Lithology descriptions from smear slides were calibrated by comparison with XRD analysis.

Standard graphic report (barrel sheet)

The LIMS2Excel application was used to extract data in a format that could be used to plot descriptive as well as instrument data in core graphic summaries using a commercial program (Strater, Golden Software). The Strater program was used to produce a simplified annotated standard graphic report, also known as a barrel sheet, for each core.

Beginning with the leftmost column, the barrel sheets display depth scale (CSF-A), core length, and section information. A fourth column displays the concatenated section-half images adjacent to a graphic lithology column in which core lithologies are represented by the graphic patterns illustrated in Figure F7. Subsequent columns provide information on drilling disturbance, sedimentary structures, lithologic accessories, and shipboard samples (Fig. F7). Additional columns present magnetic susceptibility (see “Physical properties”), color reflectance measurements, and Munsell color values.

X-ray diffraction analysis

Samples for XRD analyses were selected from working halves based on visual core observations (e.g., color variability, visual changes in lithology, and texture) and smear slides. XRD analyses were performed on three samples. Each 5–10 cm³ sample was frozen, freeze-dried in the case of unlithified samples, and ground by hand or in an agate ball mill, as necessary. Prepared samples were top-mounted onto a sample holder and analyzed using a Bruker D-4 Endeavor diffractometer mounted with a Vantec-1 detector using nickel-filtered CuKα radiation. The standard locked coupled scan was as follows:

• Voltage = 40 kV.

• Current = 40 mA.

• Goniometer scan = 5°–70°2θ.

• Step size = 0.015°2θ.

• Scan speed = 0.1 s/step.

• Divergence slit = 0.3 mm.

Diffractograms of single samples were evaluated with the Bruker DiffracPlus software package, which allowed only for mineral identification and basic peak characterization (e.g., width and maximum peak intensity). Shipboard results yielded only qualitative results on the relative occurrences and abundances of the most common mineralogical components.

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