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doi:10.2204/iodp.proc.346.102.2015

Lithostratigraphy

The primary lithostratigraphic procedures used during Expedition 346 include visual core description, sediment classification, digital color imaging, XRD, and smear slide description. Color spectrophotometry and point source magnetic susceptibility data acquired prior to core description are described in detail in “Physical properties.” Carbonate, organic matter (CHNS analysis), and geochemical measurements are described in detail in “Geochemistry.”

Core preparation

The standard method of splitting cores into working and archive halves (either using a piano wire or a saw) can affect the appearance of the split-core surface and obscure fine details of lithology and sedimentary structure. When necessary during Expedition 346, 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. Cleaned sections were then described in conjunction with measurements using the SHIL, discussed below, and SHMSL (see “Physical properties”).

Lithologic classification scheme

Sediments recovered during Expedition 346 are composed of biogenic, siliciclastic, and volcaniclastic components. They were described using a classification scheme derived from those of ODP Leg 155 (Shipboard Scientific Party, 1995), IODP Expedition 303 (Expedition 303 Scientists, 2006), IODP Expedition 339 (Expedition 339 Scientists, 2013), and Stow (2005). The biogenic component is composed of the skeletal debris of calcareous and siliceous microfauna (e.g., foraminifers and radiolarians), microflora (e.g., calcareous nannofossils, diatoms, and silicoflagellates), and macrofossil shell fragments. The siliciclastic component is composed of mineral and rock grains derived from igneous, sedimentary, and metamorphic rocks. The volcaniclastic fraction ranges from rock fragments to fine-grained tephra derived from volcanic sources. The relative proportion of these three components is used to define the major classes of sediments in this scheme (Fig. F1).

Sediment nomenclature for Expedition 346 follows the general guidelines of the ODP sediment classification scheme (Mazzullo et al., 1988), with the exception that a separate “mixed sediment” category was not used during Expedition 346. As a result, biogenic sediments are those that contain >50% biogenic grains and <50% siliciclastic/volcaniclastic grains, whereas siliciclastic/volcaniclastic sediments are those that contain >50% siliciclastic/volcaniclastic grains and <50% biogenic grains. During Expedition 346, no shallow-water biogenic materials were encountered except as accessory components; therefore, these categories are not addressed below. Sediment grain-size divisions for the siliciclastic and volcaniclastic components are based on Wentworth (1922), with nine major textural categories defined on the basis of the relative proportions of sand-, silt-, and clay-sized particles (Fig. F2), which are slightly modified from that of Expeditions 303 (Expedition 303 Scientists, 2006) and 339 (Expedition 339 Scientists, 2013). The term “clay” is only used to describe particle size and is applied to both clay minerals and all other clastic grains <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).

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. For sediment that contains >90% of one component (either the siliciclastic/volcaniclastic or biogenic component), only the principal name is used. For sediment with >90% siliciclastic components, the principal name is based on the textural characteristics of all sediment particles. For sediment with >90% volcaniclastic components, the name describes the texture as follows (Fisher and Schmincke, 1984):

• Volcanic breccia: sediment composed of pyroclasts >64 mm in diameter.

• Volcanic lapilli: sediment composed of pyroclasts between 2 and 64 mm in diameter.

• Volcanic ash: sediment composed of pyroclasts <2 mm in diameter.

During Expedition 346, volcanic breccia and volcanic lapilli were not encountered except as accessory components, and the term “tephra” was used in place of volcanic ash.

For sediment containing a greater proportion of siliciclastic grains than volcaniclastic grains and >10% volcaniclastic grains, the principal name is based on the textural characteristics of all sediment particles, with volcaniclastic components as major (25%–50%) or minor (10%–25%) modifiers (Fig. F3). For example, sediment composed of 80% detrital silty clay and 20% volcanic ash is called “silty clay with tephra,” and sediment composed of 70% detrital sandy silt and 30% volcanic ash is called “tephra-rich sandy silt.” For sediment containing a higher proportion of volcaniclastic grains than siliciclastic grains, the principal name is based on the texture of volcaniclastic grains with major or minor modifiers indicating the textural characteristics of all sediment particles. For example, sediment composed of 80% volcanic ash and 20% detrital silt is called “tephra with silt,” and sediment composed of 70% volcanic ash and 30% detrital silt is called “silty tephra.” For sediment with >90% biogenic components, the name applied indicates the most limited group of grains that exceed the 90% threshold value. For example, sediment composed of >90% calcareous nannofossils is called “nannofossil ooze,” sediment composed of 50% foraminifers and 45% calcareous nannofossils is called “calcareous ooze,” and sediment composed of 40% foraminifers, 40% calcareous nannofossils, and 15% diatoms is called “calcareous ooze with diatoms” (Fig. F1).

For sediment that contains a significant mixture of siliciclastic/volcaniclastic and biogenic components (between 25% and 75% of both siliciclastic/volcaniclastic and biogenic components), the principal name is determined by the most abundant component. If the siliciclastic component is more abundant, the principal name is based on the textural characteristics of all sediment particles (both siliciclastic/volcaniclastic and biogenic) (Fig. F1). If the volcaniclastic component is more abundant, the principal name is based on the textural characteristics of the volcaniclastic grains. If the biogenic component is more abundant, the principal name is either (1) based on the predominant biogenic component if that component forms >75% of the biogenic particles or (2) the more encompassing term “biogenic ooze.”

If one component forms 75%–90% of the sediment, then the principal name is followed by a minor modifier (e.g., “with diatoms”), with the minor modifier based on the most abundant component that forms 10%–25% of the sediment. If the minor component is biogenic, then the modifier describes the most limited group of grains that exceeds the 10% abundance threshold. If the minor component is siliciclastic, the minor modifier is based on the texture of the siliciclastic fraction.

If one component forms 50%–75% of the sediment, then the principal name is preceded by a major modifier that is based on the component that forms 25%–50% of the sediment. If the less abundant component is biogenic, then the major modifier describes the most limited group of grains that exceeds the 25% abundance threshold (e.g., nannofossil versus calcareous versus biogenic). If the less abundant component is siliciclastic, the major modifier is based on the texture of the siliciclastic fraction.

The following terms describe lithification that varies depending on the dominant composition:

• Sediment composed predominantly of calcareous, pelagic organisms (e.g., calcareous nannofossils and foraminifers):

• Ooze: sediment can be deformed with a finger.

• Chalk: sediment cannot be easily deformed manually.

• Sediment composed predominantly of siliceous microfossils (diatoms, radiolarians, and siliceous sponge spicules):

• Ooze: sediment can be deformed with a finger.

• Diatomite/Radiolarite/Spiculite: sediment cannot be easily deformed manually.

• Sediment composed of a mixture of calcareous pelagic organisms and siliceous microfossils and sediment composed of a mixture of siliceous microfossils:

• Ooze: sediment can be deformed with a finger.

• Indurated sediment: sediment cannot be easily deformed manually.

• Sediment 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 (i.e., sand, silt, or clay).

• “-stone” suffix: more consolidated material, (e.g., claystone).

• Sediment composed of sand-sized volcaniclastic grains:

• Tephra layer: sediment can be deformed easily with a finger.

• Tuff: more consolidated material.

Sediment compositions are indicated in the Graphic lithology column of the visual core description (VCD) sheets using the lithologic patterns found in Figure F4. If the primary lithology for an interval of core has a major modifier, then the graphic symbol used reflects both the major modifier and principal sediment name. The minor modifiers of sediment lithologies are not included in the Graphic lithology column.

Visual core descriptions

VCD sheets provide a summary of the data obtained during shipboard analysis of each core (Fig. F5). Detailed observations of each section were recorded using the DESClogik software, which provides data that can be used in Strater to generate a simplified, annotated graphical description (VCD) for each core. Site, hole, and depth (in meters CSF-A; previously called meters below seafloor [mbsf]) are given at the top of the VCD sheet, with the corresponding depths of core sections along the left margin (depth acronyms follow the IODP Depth Scale Terminology [www.iodp.org/​program-policies/​procedures/​guidelines/]). Columns on the VCD sheets include Lithologic unit, Core image, Graphic lithology, Tephra distribution, Coring disturbance intensity, Sedimentary structures , Lithological accessories, Bioturbation intensity, Shipboard samples, and Age. Profiles of GRA density, magnetic susceptibility, natural gamma radiation (NGR), and reflectance (L*, a*, and b*) are also included. These columns are discussed in more detail below.

Graphic lithology

Lithologies of the core intervals recovered are represented on the VCD sheets by graphic patterns in the Graphic lithology column, using the symbols illustrated in Figure F4. A maximum of two different lithologies (for interbedded sediments) can be represented within the same core interval. The major modifier of a primary lithology is shown using a modified version of the primary lithology pattern. A secondary lithology present as interbeds within the primary lithology is shown by a pattern along the right side of the column, with a solid vertical line dividing the primary and secondary 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 written core description at the top of the VCD. However, some distinctive secondary lithologies, such as tephra layers, are included graphically in the Graphic lithology column as the primary lithology for a thin stratigraphic interval. Relative abundances of lithologies reported in this way are useful for general characterization of the sediment but do not constitute precise, quantitative observations.

Lithologic accessories

Lithologic, diagenetic, and paleontologic accessories, such as nodules, sulfides, and shells, are indicated on the VCD sheets. The symbols used to designate these features are shown in Figure F6. The following terminology was used to describe the abundance of lithologic accessories in written core descriptions:

• Trace = 1 observed per section of core.

• Rare = 2–10 observed per section of core.

• Common = 10–20 observed per section of core.

• Abundant = 20–50 observed per section of core.

• Dominant = >50 observed per section of core.

Clasts

When clasts >2 mm were present, this was noted in core descriptions using the same abundance terminology as for lithologic accessories (e.g., trace, rare, common, abundant, and dominant). Where only holes or depressions caused by clasts were observed, the working half was also examined to better estimate clast abundance. Details on the lithology and shape of large lonestones and diamict clasts are provided on the written core descriptions and/or the DESClogik General interval comments column.

Tephra type

Occurrence of tephra layers is recorded in an additional column on the VCD sheets using the symbols shown in Figure F6. The type of tephra is defined visually and classified as

• V = vitric (primarily volcanic glass shards).

• P = pumice (white to yellowish pumice grains).

• S = scoria (black–dark gray scoria grains).

Characteristics of tephra layers, such as grain size, color, and sedimentary structures and characteristics of their components, such as glass type (bubble-walled, pumice-walled, or fibrous), glass morphology, associated heavy minerals, and rock fragments, were recorded.

Bioturbation

Four levels of bioturbation are recognized using a scheme similar to that of Droser and Bottjer (1986). Bioturbation intensity is classified as

• 1 = none.

• 2 = slight.

• 3 = heavy.

• 4 = complete.

These levels are illustrated by numeric scale in the Bioturbation intensity column of the VCD sheet. Recognizable biogenic structures and trace fossils were additionally noted.

Stratification and sedimentary structures

The locations and types of stratification and sedimentary structures visible on the prepared surfaces of the split cores are shown in the Sedimentary structures column of the VCD sheet. Symbols in this column indicate the locations and scales of stratification, as well as the locations of individual bedding features and any other sedimentary features, such as scours, ripple laminations, and fining-upward, coarsening-upward, or bigradationally bedded intervals (Fig. F6).

In the written description, layers and bedding thickness were further described and classified following terminology based on Stow (2005):

• Thin lamination = <3 mm thick.

• Medium lamination = 0.3–0.6 cm thick.

• Thick lamination = 0.6–1 cm thick.

• Very thin bed = 1–3 cm thick.

• Thin bed = 3–10 cm thick.

• Medium bed = 10–30 cm thick.

• Thick bed = 30–100 cm thick.

• Very thick bed = >100 cm thick.

Descriptive terms for bed boundaries, such as sharp, erosive, gradual, irregular, and bioturbated, are noted in DESClogik.

Sediment disturbance

Core disturbance from the drilling process can impact the integrity of the stratigraphic sequence. Drilling disturbance, if any, is documented for both soft and firm sediment using the following classification scheme:

• Slightly disturbed: bedding contacts are slightly bent or bowed in a concave-downward appearance.

• Moderately disturbed: bedding is moderately deformed but probably still in the correct stratigraphic sequence.

• Heavily disturbed: sediment is completely deformed and may show no traces of original bedding or structure.

In addition to this first-order assessment of disturbance, a number of other terms (Fig. F6) may appear on the VCD to characterize drilling disturbance. Some of the more common types observed include

• Flow-in: soupy, displaced sediment pulled into the core liner during APC coring.

• Fall-in: downhole contamination resulting from loose material falling from the drill hole walls into the top of the core. The uppermost 10–15 cm of each core was inspected during description for potential fall-in.

• Bowed: bedding contacts are slightly to moderately deformed but still subhorizontal and continuous.

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

• Biscuit: sediment of intermediate stiffness shows vertical variations in the degree of disturbance. Softer intervals are washed and/or soupy, whereas firmer intervals are relatively undisturbed.

• Cracked or fractured: firm sediment is broken but not displaced or rotated significantly.

• Fragmented or brecciated: firm sediment is pervasively broken and may be displaced or rotated.

Shipboard samples

Sample material taken for shipboard sedimentologic and chemical analyses consisted of interstitial water whole rounds and Rhizon samples, microbiology whole rounds and syringes, micropaleontology samples, smear slides, discrete samples for XRD and carbonate analysis, and samples for physical properties (moisture and density [MAD]) and paleomagnetic studies. In addition, a micropaleontology sample was obtained from the core catcher of most cores. XRD samples for bulk analysis were routinely taken from Section 1 of each core adjacent to the paleomagnetic sample and at other levels of lithologic interest. Carbonate analyses were routinely taken from interstitial water squeeze cakes at the rate of two per core and at additional levels where requested. Thick (>1 cm) tephra layers were also sampled for the establishment of age models and correlations between holes and cores.

Color

During Expedition 346, the dominant sediment colors for each core are recorded in the summary at the top of the VCD sheets using standard Munsell color names (Munsell Color Company, Inc., 2009).

Remarks

The written description at the top of the VCD sheets for each core contains a summary of primary and secondary lithologies present, as well as notable features such as sedimentary structures, grading, and disturbances resulting from the coring process.

Smear slides

Smear slide samples were taken from archive halves during core description when there was either a major lithologic change or tephra layer. A small amount of sediment was removed with a wooden toothpick, dispersed evenly in deionized water on a 25 mm × 75 mm glass slide, and dried on a hot plate at a low setting. A drop of mounting medium (Norland Optical) and a 22 mm × 30 mm cover glass were added, and the slide was placed in an ultraviolet (UV) light box for ~15 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. An eyepiece micrometer was calibrated once for each magnification and combination of ocular and objective, using an inscribed stage micrometer.

Relative abundance (percent) of each grain size and type was 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/volcaniclastic sand) because these are difficult to incorporate into the smear. Biogenic silica, which is transparent and isotropic, can also be difficult to quantify. After scanning for grain-size distribution, several fields were examined at 200×–500× for mineralogic and microfossil identification.

Standard petrographic techniques were employed to identify commonly occurring minerals and biogenic groups, as well as important accessory minerals and microfossils. The smear slide analysis data worksheet used during these analyses is shown in (Fig. F7), and the data generated are included in the core descriptions. These tables provide information about the sample location, description of where the smear slide was taken, the estimated abundances of texture (i.e., sand, silt, and clay), and the relative composition of individual components in the sediment (i.e., tephra, siliciclastics, detrital carbonate, biogenic carbonate, and biogenic silica). Relative abundances of identified components such as mineral grains, microfossils, and biogenic fragments were assigned on a semiquantitative basis using the following abbreviations:

• Tr = trace (<1% in field of view [FOV]).

• R = rare (1%–5% in FOV).

• C = common (5%–25% in FOV).

• A = abundant (25%–75% in FOV).

• D = dominant (>75% in FOV).

In addition, an assessment of fossil preservation was made using the following abbreviations:

• P = poor.

• M = moderate.

• G = good.

Digital color imaging

The SHIL captures continuous high-resolution images of the archive-half surface for analysis and description. Images were collected shortly after core splitting and surface scraping in an effort to avoid color changes resulting from excessive sediment drying and oxidation of the surface. The shipboard system uses a commercial line-scan camera lens (AF Micro Nikon; 60 mm; 1:2.8 D), with illumination provided by a custom assembly of three pairs of LED strip lights that provide constant illumination over a range of surface elevations. Each LED pair has a color temperature of 6,500 K and emits 90,000 lux at 3 inches. The resolution of the line-scan camera was set at 20 pixels/mm. Available files include the original high-resolution TIFF image with a grayscale and ruler, as well as reduced JPEG images cropped to show only section-half surfaces. High-resolution color data (red, green, and blue) were acquired by digital color imaging.

X-ray diffraction analysis

Samples were prepared for XRD analysis in order to make qualitative to semiquantitative bulk mineral estimates. The XRD results combined with smear slide estimates and visual descriptions were used to assist in lithologic classification. In general, one 2.5 cm³ sample was routinely taken for analysis in Section 1 of each core for APC samples or every two cores for half APC samples adjacent to the paleomagnetic sample. Additional limited samples were taken and analyzed based on visual core observations (e.g., color variability and visual changes in lithology and texture) and smear slides. Samples analyzed for bulk mineralogy were freeze-dried in the case of unlithified samples and ground by hand (soft sediment) or in an agate ball mill (rock), 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 settings were as follows:

• Voltage = 40 kV.

• Current = 40 mA.

• Goniometer scan = 4°–70°2θ.

• Step size = 0.0087°2θ.

• Scan speed = 0.2 s/step.

• Divergence slit = 0.3 mm.

Shipboard results yielded only qualitative results of the presence and relative abundances of the most common mineral components.

Diffractograms of bulk samples were evaluated with the aid of the EVA software package, which allowed for mineral identification and basic peak characterization (e.g., baseline removal and maximum peak intensity). Files were created that contained d-spacing values, diffraction angles, and peak intensities with background removed. These files were scanned by the EVA software to find d-spacing values characteristic of a limited range of minerals, occasionally using aluminum oxide as an external standard to monitor data quality. Peak intensities were reported for each mineral identified to provide a semiquantitative measure of how each mineral identified varied downhole and among sites. Opal-A peak height was determined by the height of its peak at 22°2θ relative to the background level (Tada and Iijima, 1992), which is defined as the average of the intensities at 19° and 29°2θ. The abundance of opal-CT was measured by the height of the peak at 22°2θ when only typical opal-CT peaks occurred. Muscovite/illite and kaolinite/chlorite have similar diffraction patterns and were usually not distinguished shipboard. Digital files with the diffraction patterns are available from the LIMS database (iodp.tamu.edu/​tasapps/).

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