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doi:10.2204/iodp.proc.324.102.2010

Sedimentology

The lithostratigraphy of sediments recovered during Expedition 324 was determined using a variety of descriptive and analytical techniques. These included digital core imaging, color reflectance, magnetic susceptibility, and sedimentological observations based on visual (macroscopic) core description, thin sections, and smear slides. The methods employed during this expedition were similar to those used during Ocean Drilling Program (ODP) Leg 198 (Shipboard Scientific Party, 2002a), during which sediments were also drilled on Shatsky Rise. Expedition 324 used the DESClogik application (see link to user guides at iodp.tamu.edu/tasapps/) to record descriptive data for the Laboratory Information Management System (LIMS) database, which was first implemented during IODP Expedition 320T (Expedition 320T Scientists, 2009). Three spreadsheet templates were customized for Expedition 324 in DESClogik prior to the arrival of the first core. The spreadsheet templates were used to record macroscopic core descriptions, as well as smear slide and thin section data, which helped quantify the texture and relative abundance of biogenic and nonbiogenic components. The locations of all smear slide and thin section samples taken from each core were recorded in the IODP-USIO Sample Master program. The descriptive data were used to produce the visual core description (VCD) graphic reports.

Core imaging

After whole cores were split into archive and working halves, the flat face of the archive half was imaged using the SHIL. Images were taken with a line scan camera at 20 pixel/mm intervals. This process generated high-resolution digital images that aided core description, as the high quality of the images enhances the appearance of some sedimentary features.

Visual core descriptions

Cores were examined and the descriptions were entered into the DESClogik sedimentary template, from where they were uploaded into the central LIMS database. The data were used to produce VCD graphic reports (Fig. F1), which include simplified graphical representations of the cores with accompanying descriptions of the features observed. Depending on the type of material drilled, two VCDs were sometimes produced for each core, one to describe the sediments/sedimentary rocks recovered and the other for description of igneous features. In contrast to the igneous VCD, which displays data in terms of core sections, the sedimentary VCD shows the entire core. For further information about the igneous VCDs, see "Igneous petrology."

Hole, core, and interval (core depth below seafloor) are given at the top of the VCD, and core depths and section lengths are indicated along the far left margin. Physical descriptions of the core correspond to entries in the DESClogik application and include lithology, grain size, sedimentary structures, bioturbation, accessory features, and the types of samples taken from the core for shipboard studies. Additionally, selected physical property data (gamma ray attenuation [GRA] bulk density, magnetic susceptibility, and color reflectance) and paleontological observations are displayed on the sediment VCDs. A core summary section at the top of the VCD provides a generalized overview of the lithology and features of the whole core. The individual columns shown in the VCD are listed below, followed by an outline of the lithostratigraphic classification used during Expedition 324.

Core summary

A brief overview of major and minor lithologies present in the core, as well as notable features (e.g., sedimentary structures), are presented in the core summary text field at the top of the VCD. The summary includes sediment color determined qualitatively using the Munsell soil color charts (Munsell Color Company, Inc., 1994). Because color of the sediment may evolve during drying and subsequent redox reactions, color was described shortly after the cores were split.

Section

Once on deck each core was cut into sections. The section number is listed in the far left column of the VCD along the core image scan and the core depth in meters.

Core image

The high-resolution scans of each core section made by the SHIL are compiled to provide a continuous image of the entire core to the right of the column listing the core sections. Sedimentary VCDs are created only for cores which contain at least a 1 m interval of sediments.

Graphic lithology

The lithology of the core recovered is represented on the VCD sheet by graphic patterns (Fig. F2). For intervals containing homogeneous mixtures of multiple lithologies, patterns are arranged within the column from left to right in order of their relative abundance. Only components determined to be the primary lithology of the sediment (most abundant) are represented in the Graphic Lithology column. The width of each pattern in the column gives the relative abundance of that component. Therefore, the Graphic Lithology column represents an approximate estimate of the sedimentary constituents, rather than a quantitative characterization. It is important to note that occasionally other sedimentary constituents, which comprise >25% (major component), are not displayed. We suggest interested parties access the LIMS database (iodp.tamu.edu/tasapps/) to obtain detailed descriptive information and a complete characterization of the sedimentary material recovered.

Gamma ray attenuation bulk density

Where recovery and core length permitted, the bulk density of the sediments was measured using GRA (see "Physical properties"). The raw and filtered data appear in the first column to the right of the Graphic lithology column.

Magnetic susceptibility

Where recovery and core length permitted, the magnetic susceptibility, which roughly indicates sedimentary iron content, was measured (see "Physical properties"). The raw and filtered data appear beside the GRA bulk density column.

Color reflectance spectrophotometry

Reflectance of visible light from the archive halves of sediment cores was routinely measured using a SHMSL. The SHMSL was equipped with OceanOptics software for analysis of color reflectance data. Cores consisting of soft sediment were covered with clear plastic wrap and placed on the SHMSL. Hard rock sections were run without the protective plastic cover. Measurements were taken at 1–2 cm spacing. The SHMSL is set to skip empty intervals in the core liner but cannot recognize relatively small cracks, disturbed areas of core, or plastic section dividers. Thus, raw SHMSL data may contain spurious measurements that should ideally be edited out of the data set before use. Therefore, a filter was applied to the reflective data set (see "Physical properties"). Both filtered and unfiltered data are shown on the VCD.

Age

The nannofossil or foraminifer zone which defines the age of the sediments is listed in the Age column.

Grain size

The Grain size column displays the dominant grain size of the sediment. Where mixtures of multiple sedimentary constituents with varying grain sizes occur, the most prevalent grain size is displayed. Rather than being plotted by absolute size, sedimentary grain sizes are represented numerically using a range from 0 (cryptocrystalline) to 6 (gravel/pebble/lapilli) (Fig. F2).

Shipboard sample

Samples taken for shipboard analyses are recorded here. These samples include toothpick samples and thin sections, all micropaleontology samples, and those taken for physical properties measurements. At least one or two smear slides (or thin sections) were generated from each core to constrain sedimentary composition. In addition, coulometric analyses of carbonate content and carbon-hydrogen-nitrogen-sulfur (CHNS) analyses of total sedimentary carbon were performed on discrete samples to aid lithologic description.

Bioturbation

Degree of bioturbation was determined by observing how intensely the sediment was altered by the action of organisms (Droser and Bottjer, 1986). Degree of bioturbation was determined based on the following parameters: intense (75%–100%), moderate (25% to <75%), and minor (>0% to <25%). Graphic symbols in the Bioturbation intensity column illustrate these parameters (Fig. F2).

Fossils

Identifiable trace fossils (ichnofossils) and any major body fossils or fossil fragments recovered are displayed to the right of the Bioturbation intensity column in the Ichnofossil/Fossil column.

Structures

Structures resulting from physical sedimentary processes and those associated with drilling disturbance are shown in the following columns.

Sedimentary structure

Structures formed by both physical and biogenic processes are included. These processes include detailed descriptions of features such as laminations and cross-bedding, contacts between sediments of differing lithologies, and any soft-sediment deformation structures (e.g., flame structures).

Lithologic accessories

Nonprimary sedimentary constituents such as authigenic or diagenetic minerals are recorded in the Lithologic accessories column. Alteration structures and postdepositional features (e.g., sedimentary coatings) also appear here.

Drilling disturbance

The degree to which the core is affected by drilling disturbance is recorded in this column (e.g., biscuiting) (Fig. F2).

Sediment classification

Lithologic names including sediment composition, degree of lithification, and/or texture are based on conventions outlined by Mazzullo et al. (1988) in the ODP sediment classification scheme. The principal name is based on the composition of the major lithology as identified by observation, smear slides, and so on, and is preceded by major modifiers (in order of increasing abundance) that refer to components making up at least 25% of the sediment. Minor components represent between 10% and 25% of the sediment and follow the principal name in order of increasing abundance. Thus a well-indurated sample containing 70% nannofossils, 25% clay minerals, and 5% foraminifers would be described as a "clayey nannofossil limestone." Only the major lithology is recorded in the Graphic lithology column, whereas other major and minor components are available from the LIMS database.

Size divisions for grains are those of Wentworth (1922) (Fig. F3). These size and textural qualifiers were not used in the sediment names (e.g., nannofossil clay implies that the dominant component is clay minerals rather than clay-sized nannofossils).

Terms that describe lithification are dependent upon the dominant composition. For sediments derived predominantly from calcareous organisms (e.g., calcareous nannofossils and foraminifers) the terms "ooze," "chalk," and "limestone" are applicable. If the sediment can be deformed with a finger it is classed as ooze, if it can be scratched easily by a fingernail it is chalk, and if it cannot be scratched easily it is limestone.

For sediments derived predominantly from siliceous microfossils (diatoms, radiolarians, and siliceous sponge spicules) the terms "ooze," "radiolarite/spiculite/diatomite," and "chert" are applicable. If the sediment can be deformed with a finger it is classified as ooze, if it cannot be easily deformed manually it is radiolarite/spiculite/diatomite, and if it displays a glassy luster it is chert. The term "porcellanite" is used as defined by Keene (1975) and describes a siliceous limestone/claystone that has a dull luster and is less hard and compact than chert. Porcellanite 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.

For sediments derived predominantly from siliciclastic material, naming is based on the dominant grain size rather than the degree of lithification. For indurated material, the lithification suffix "-stone" is appended to the dominant size classification (e.g., "silt" versus "siltstone").

For sediments composed predominantly of fine-grained volcaniclastic grains the terms "volcanic ash" and "tuff" are applicable. If the sediment can be deformed easily with a finger, the interval is described as ash and a modifier of "coarse" or "fine" is given to indicate the grain size range of sand and silt/clay, respectively. For more consolidated material, the rock is called tuff. The term "volcaniclastic breccia" is used for coarse-grained material (i.e., granule and pebble size categories). Discrete pumice lapilli are noted simply as "pumice." If a rock is composed of volcanic grains which have obviously experienced transport and redeposition it may be referred to as a "volcaniclastic sandstone" or "volcaniclastic mudstone." The terms "breccia" and "conglomerate" are used for rocks which contain many grains >2 mm in size. If the composition of the majority of the clasts is igneous, then "volcaniclastic" may be added to the beginning as a descriptive qualifier. At Site U1348, the term "hyaloclastite" is used for vitroclastic (i.e., glassy) materials produced by the interaction of water and hot magma or lava (Fisher and Schmincke, 1984). Fine-grained hyaloclastite is classified into "fine hyaloclastite" (clay and silt size categories) and "hyaloclastite sandstone" (sand size categories). The terms "granular hyaloclastite" and "hyaloclastite breccia" are used for rocks that contain many grains 2–4 mm and >4 mm in size, respectively. The use of genetic nomenclature for the volcaniclastic succession recovered at Site U1348 is based on the assumption that the material recovered is predominantly (altered) hyaloclastite (Table T1). This assumption however, must be verified by further detailed studies.

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