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doi:10.2204/iodp.proc.323.102.2011

Lithostratigraphy

In the description of marine sediments collected by IODP and its predecessors, lithologic, textural, and genetic sediment classifications have often been used interchangeably. Moreover, criteria that range from purely descriptive to purely interpretational have been used to identify sedimentary structures. This has been a significant obstacle to the objective representation of sediment characteristics and has complicated the exchange of information between marine sedimentologists and other marine scientists and, more importantly, the interpretation of sediment data. Consequently, we sought to document the sediments collected during Expedition 323 with objective, reproducible, and unbiased descriptions. Interpretation was only employed for primary (e.g., cross-lamination and turbidites) and nonprimary (e.g., drilling disturbances) sedimentary and diagenetic structures that unequivocally meet the criteria explained in this section.

Preparation for core description

After the core sections were split, the archive halves were put on the description table. Before further processing occurred, the split surfaces of the archive halves were scraped lightly with a glass slide or spatula to create an even surface.

Smear slides

One or more smear slide samples of the main lithology were collected from the archive half of each core section. Additional samples were collected from areas of interest (e.g., laminations, ash layers, mottles, etc.). A small amount of sediment was taken with a wooden toothpick and put on a 2.5 cm × 7.5 cm glass slide. The sediment sample was homogenized with a drop of deionized water and evenly spread across the slide to create a very thin (about <50 µm) uniform layer of sediment grains for quantification. The dispersed sample was dried on a hot plate. A drop of Norland optical adhesive was added as a mounting medium to a coverslip, which was carefully placed on the dried sample to prevent air bubbles from being trapped in the adhesive. The smear slide was then fixed in an ultraviolet light box.

Smear slides were examined with a transmitted-light petrographic microscope equipped with a standard eyepiece micrometer. The texture of siliciclastic grains (relative abundance of sand-, silt-, and clay-sized grains) and the proportions and presence of biogenic and mineral components were recorded on a smear slide sample sheet (Fig. F1). Biogenic and mineral components were identified, and their percentage abundances were visually estimated using Rothwell (1989). The mineralogy of clay-sized grains could not be determined from smear slides. Note that smear slide analyses tend to underestimate the amount of sand-sized and larger grains because these grains are difficult to incorporate onto the slide. Postcruise quantitative analysis of grain sizes from Site U1339 demonstrated that clay-sized grains were underdescribed in the smear slides (I. Aiello, pers. comm., 2010), and this is probably the case for slides from other sites as well.

Thin sections

To better characterize the composition and texture of clasts, very sandy lithologies, relatively large (>2 cm) authigenic nodules, and other similar lithified and partially lithified structures, petrographic thin sections (~30 µm thick) were prepared and then analyzed with a petrographic microscope. The observations were recorded on a thin section sample sheet (Fig. F2).

Digital color imaging

The flat faces of the archive halves were scanned with the SHIL as soon as possible after splitting and scraping to avoid color changes caused by sediment oxidation and drying. The SHIL uses three pairs of advanced illumination high-current-focused light-emitting diode (LED) line lights to illuminate large cracks and blocks in the core surface and sidewalls. Each LED pair has a color temperature of 6500 K and emits 90,000 lx at 3 inches. A line-scan camera imaged 10 lines per millimeter to create a high-resolution TIFF file. The camera height was adjusted so that each pixel imaged a 0.1 mm² section of the core. However, actual core width per pixel varied some because of differences in section-half surface height. High- and low-resolution JPEG files were subsequently created from the high-resolution TIFF file. All image files include a grayscale and ruler. Section-half depths were recorded so that these images could be used for core description and analysis.

Spectrophotometry (color reflectance)

Color reflectance was measured on the surfaces of the split cores every 5 cm using the SHMSL. The surfaces of the archive halves were covered with plastic wrap to prevent contamination of the spectrophotometer lens. The Ocean Optics USB4000 spectrophotometer recorded counts every ~0.2 nm in the range of 177 to 925 nm, which includes the visible and near-infrared spectra. Spectral data were related to color in a cylindrical coordinate system using the tristimulus values L*, a*, and b* (Commission Internationale d'Eclairage, 1986), where the axis of the cylinder, L*, is total light reflected, or luminosity, a* is redness, and b* is yellowness. Before each core was measured, the spectrophotometer was calibrated with a Labsphere-certified white reflectance standard and a black light trap with the light source shuttered. For Sites U1342–U1345, the standards were covered with the same plastic wrap used to cover the core sections. For Sites U1339–U1341, the instrument was calibrated every ~12 h with the same piece of plastic wrap covering the standards and different plastic wrap covering the core. The reflectance data contain artifacts from the leakage of room lights to the sensor as a function of the flatness and the induration of the sediment surface. Secular drift in the data occurred with infrequent calibration, possibly precluding the use of these data for quantitative reconstruction of sediment composition.

X-ray diffraction analysis

Intervals of interest (e.g., marked lithologic or color contrasts, diagenetic layers or nodules, and microscopically unidentified minerals) identified during visual core description and in smear slides were sampled for mineralogical analyses from the working halves of the cores. Minimum sample volumes of ~5 cm³ were frozen, freeze-dried, and ground by hand or in an agate ball mill. Prepared samples were mounted onto a sample holder and analyzed by XRD. XRD samples were 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° to 90°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. Reliable results obtained by this analysis are limited to minerals composing at least 1% of the total sediment. Notably, quantification of mineral content was not possible because the samples were not spiked with a defined amount of mineral standard for calibration. Therefore, shipboard results yield only qualitative results on the relative occurrences and abundances of the most abundant mineralogical components. XRD spectra and interpretations are available as supplementary materials (see XRD in "Supplementary material").

Sediment description worksheets, standard graphical reports, and summary figures

Sediment lithology, color (using Munsell soil color charts [Munsell Color Company, 1994]), structures, accessories, disturbances, and other observations were recorded on visual core description worksheets (Figs. F3, F4). The handwritten core description worksheets are available as supplementary material (see LITH in "Supplementary material"). In addition, core descriptions were entered into the Laboratory Information Management System (LIMS) database using DESClogik. There are some differences between the core description worksheets and the data in LIMS, so in case of disagreement, the core description worksheets should be considered the primary record of core description. The LIMS database was used to make two types of core description summaries.

First, annotated standard graphical reports, or visual core descriptions (VCDs), were created for each core to summarize data collected during Expedition 323 (Fig. F5). The Strater software package was used to compile lithology, biostratigraphy, and physical properties data entered into DESClogik. Each VCD includes site, hole, and core number, followed by a narrative description of major and minor lithologies, the types of boundaries observed between lithologies, and color. On the far left of the VCD are depth below seafloor (mbsf), core length, and section breaks, which provide vertical reference points. The scanned core image is followed by a graphical representation of lithologies and contacts using the colors, patterns, and symbols illustrated in Figure F6. Single or multiple interbedded lithologies can be represented in one interval. In general, only lithologies composing >25% of the core are shown graphically; however, an exception is made for some distinct minor lithologies such as ash or authigenic carbonate layers. The right-hand columns show bioturbation intensity, coring disturbances, sedimentary structures, lithologic accessories, samples taken during Expedition 323 for shipboard analyses (e.g., XRD, interstitial water, etc.), and stratigraphic ages (datums). Further to the right are plots of physical properties data, including magnetic susceptibility (10^–5 SI units), gamma ray attenuation (GRA) bulk density (g/cm³), and color reflectance parameter b* (blue to yellow color space). The far right column contains comments such as notes about laminations, authigenic carbonates, or unusual structures and accessories.

Second, lithologic summary figures were produced for each hole. These figures show 250 m per page (in contrast to the single core [~10 m] per page shown in the VCDs), except for Site U1342 summary figures, which show only 65 m per page (Fig. F7). These figures include depth below seafloor (mbsf), core recovery, a lithologic column (which uses the same colors as the VCDs but does not include symbols or contacts), and unit boundaries. Also plotted are the occurrences of soft-sediment deformation, coring disturbances, ash layers, lithologies with >25% sand, sandy layers and mottles, clasts, shells, authigenic carbonates (including yellowish foraminifers recorded by biostratigraphers; see "Biostratigraphy" and "Lithostratigraphy" in the "Site U1343" chapter), and the abundances of diatoms, foraminifers, calcareous nannofossils, and sponge spicules (Fig. F7). To the right of these are plots of color reflectance parameter b*, natural gamma radiation (NGR), GRA bulk density, and magnetic susceptibility (plotted with lines showing the data averaged in 0.25 m nonoverlapping bins), followed by selected biostratigraphic and paleomagnetic datums. The Lisiecki and Raymo (2005) oxygen isotope stack is plotted in the panel on the far right, with datum ages and paleomagnetic ages provided for comparison.

Sediment and hard rock classification

Different lithologic classifications have been proposed for ODP and IODP cruises during the last decade, and these classifications have been adapted according to cruise objectives and/or specific goals of shipboard participants. In some cases, these classifications are inconsistent with other mineralogical and geochemical determinations of the same data set. Because the goal of classifying sediments during Expedition 323 was mainly compositional, the use of genetic terms such as "pelagic" was avoided and the term "hemipelagic" was replaced with "mixed" to describe sediments with mixed biogenic and siliciclastic composition.

Sediment lithology

The principal name applied to a sediment is determined by the component or group of components (e.g., total biogenic silica) making up >60% of the sediment or rock, with the exception of subequal mixtures of biogenic and siliciclastic and/or volcaniclastic material (Fig. F8D). Principal names are determined as follows.

Siliciclastic sediments

If total siliciclastic plus volcaniclastic content is >60% (Fig. F8D) and siliciclastic content is >80% of the siliciclastic plus volcaniclastic fraction (Fig. F8C), the principal name is determined by the texture of the siliciclastic grains, that is, the relative proportions of sand-, silt-, and clay-sized grains when plotted on a modified Shepard (1954) ternary classification diagram (Fig. F8A). The siliciclastic principal names are clay, silt, sand, silty clay, sandy clay, clayey silt, sandy silt, clayey sand, and silty sand.

Volcaniclastic sediments

Volcaniclastic sediments are defined as sediments derived from a primary volcanic process, including epiclastic sediment, the particles of which are derived from the weathering and erosion of preexisting volcanic rock, and primary volcanic material. Primary volcanic processes include sediment derived from the quench-granulation of lava flows as well as pyroclastic processes such as air fall and pyroclastic flow. If total siliciclastic plus volcaniclastic content is >60% (Fig. F8D) and volcaniclastic content is >80% of the siliciclastic plus volcaniclastic fraction (Fig. F8C), volcanic particles are designated as fine ash (<0.063 mm), coarse ash (0.063–2 mm), or lapilli (2–64 mm) according to Fisher and Schmincke's (1984) and Gillespie and Styles's (www.bgs.ac.uk/downloads/start.cfm?id=7) classifications. Principal names are then given according to the category of the majority of the volcaniclastic grains. If a primary volcanic nature can be ascertained, the sediment is described using the conventions employed for siliciclastic sediment.

Biogenic sediments

If total biogenic content is >60% (i.e., siliciclastic plus volcaniclastic material is <40%), the principal name applied is ooze (Fig. F8B). The major modifier consists of the name(s) of the major fossil group(s) composing at least 40% of the biogenic fraction, with the least common fossil listed first. Biogenic components are not described in textural terms. Thus, sediment containing 65% sand-sized foraminifers and 35% siliciclastic clay is called foraminifer ooze, not foraminifer sand.

Mixed sediments

Mixed sediments include sediments with <40% biogenic grains and a subequal mixture of siliciclastic and volcaniclastic grains or sediments with 40%–60% biogenic grains. The principal names of mixed sediments have two parts, as described below.

If total siliciclastic plus volcaniclastic content is >60% (Fig. F8D) and siliciclastic content is 50%–80% of the siliciclastic plus volcaniclastic fraction (Fig. F8C), the principal name is determined by the texture of the siliciclastic grains, with the major modifier determined by the size of the volcaniclastic grains (as above). For example, the principal name of a sample containing 20% diatoms, 30% fine ash, and 50% lithogenic grains that are >75% silt-sized is fine-ashy silt.

If total siliciclastic plus volcaniclastic content is >60% (Fig. F8D) and volcaniclastic content is 50%–80% of the siliciclastic plus volcaniclastic fraction (Fig. F8C), the principal name is determined by the size of the volcaniclastic fraction (as above), with the major modifier determined by the texture of the siliciclastic grains (Fig. F8A). For example, the principal name of a sample containing 20% diatoms, 30% lithogenic grains that are >75% silt-sized, and 50% fine ash is a silty fine ash.

If biogenic content is 40%–60%, (Fig. F8D), the principal name consists of two parts: (1) the principal name appropriate for the siliciclastic components (if there is a greater proportion of siliciclastic than volcaniclastic grains) or the principal name appropriate for the volcaniclastic components (if there is a greater proportion of volcaniclastic than siliciclastic grains) and (2) a major modifier appropriate for the major fossil group(s) (as above). Examples of principal names for mixed sediments are diatom fine ash and foraminifer-diatom silty clay.

Prefixes

If a microfossil group composes 5%–40% of the sediment and this group is not included as part of the principal name, minor modifiers are used. When a microfossil group (e.g., diatom, nannofossil, or foraminifer) composes 10%–40% of the sediment, a minor modifier (see Fig. F8B) consisting of the component name hyphenated with the suffix "rich" (e.g., diatom-rich clay) is used. When a microfossil group composes 5%–10% of the sediment, a minor modifier (see Fig. F8B, F8D) consisting of the component name hyphenated with the word "bearing" (e.g., diatom-bearing clay) is used. When two minor components are present, minor modifiers are listed before the principal name in order of increasing abundance. For example, sediment with 15% foraminifers, 40% nannofossils, and 45% clay is foraminifer-rich nannofossil clay; sediment with 5% diatoms, 15% radiolarians, and 80% clay is diatom-bearing radiolarian-rich clay.

Indurated lithologies

Hard indurated rocks have names that reflect the major constituent of the sediment in which they originated:

• Dolostone: a white indurated rock composed of authigenic dolomite;

• Claystone: an indurated rock composed of clay;

• Siltstone: an indurated rock composed of silt;

• Sandstone: an indurated rock composed of sand;

• Breccia: an indurated rock composed of larger than sand-sized angular clasts;

• Fine ash tuff: an indurated rock composed of fine ash–sized primary volcanic particles;

• Coarse ash tuff: an indurated rock composed of coarse ash–sized primary volcanic particles;

• Lapilli tuff: an indurated rock composed of lapilli-sized primary volcanic particles;

• Tuff-breccia: an indurated rock composed of block-sized and block-shaped particles in a matrix of ash-sized primary volcanic particles;

• Agglomerate: an indurated rock composed of bomb-sized and bomb-shaped primary volcanic particles;

• Volcaniclastic sandstone: an indurated rock composed of sand-sized volcanic particles (this name may be modified by indicating the size of the sand particles—e.g., a volcaniclastic medium-grained sandstone); and

• Volcaniclastic breccia: an indurated rock composed of gravel-sized volcanic particles.

Induration is separated into four classes:

1. Soupy: water-saturated sediment with very little strength;

2. Soft: sediment with little strength that is readily deformed under the pressure of a finger or broad-blade spatula;

3. Stiff: partly lithified sediment that is readily scratched with a fingernail or the edge of a spatula; and

4. Hard: well-lithified and cemented sediment that is resistant or impossible to scratch with a fingernail or the edge of a spatula or core that must be cut with a band saw or diamond saw.

Gravel-sized grains

Several sites drilled during Expedition 323 are located along the glacially influenced continental margin of Alaska. Isolated gravel-sized clasts were recorded and, when possible, are described as follows. Note that clast occurrence is almost certainly undersampled because our observations were limited to the cut surface of the section halves. Clasts are classified as sedimentary, igneous, or metamorphic and are described according to their roundness and sphericity (Powers, 1953). Sedimentary rocks are described by color (Munsell chart) and grain size of the groundmass and are given a name consistent with the indurated lithologies described above. Metamorphic rocks are classified as either nonfoliated or foliated. Nonfoliated rocks are called "hornfels." Foliated rocks are classified as slate, phyllite, schist, or gneiss depending on the nature of the foliation. Igneous rocks are described by color (Munsell chart) and, when possible, phenocryst abundance (in percent or number/inch²), phenocryst composition, phenocryst grain size, vesicle abundance, and vesicle shape (spherical, ovoid, and irregular) and are named following the scheme of Streckeisen (1974).

Hard rocks at Site U1342

Hard rocks recovered from Site U1342 are described in the same way as gravel-sized grains (above). Volcaniclastic rocks drilled at Site U1342 are additionally described by the grain size of the groundmass, the shape of the clasts or grains, the degree of rounding of the clasts or grains, and the degree of sorting (including bimodal sorting and sorting ranging from very well sorted to very poorly sorted). Igneous rocks are further described by phenocryst abundance (in percent or number/inch²), phenocryst composition, phenocryst grain size, vesicle abundance, composition of vesicle infilling, and vesicle shape (spherical, ovoid, and irregular), following the scheme of Streckeisen (1974).

Accessories

Accessories include macroscopic biogenic remains such as shells, sponge spicule aggregates, worm tubes, wood fragments, and mottling (e.g., ash, sand, and pyrite), as well as clasts, concretions, nodules, alteration halos, specks, sandy layers, and ash layers (<2 cm thick). A concretion is a small, irregularly rounded knot, mass, or lump of a mineral or mineral aggregate, normally having a warty or knobby surface and no internal structure and usually exhibiting a contrasting composition from the sediment or rock matrix in which it is embedded. A nodule is a regular, globular structure. When possible, clasts, concretions, and nodules are described by composition. An alteration halo is an area where sediment is a different color or composition in a ring surrounding a grain or accessory phase. A speck is a small spot or smear where sediment is a different color or composition than the surrounding sediment (it is not ring shaped, like an alteration halo). Pyrite is an example of a speck composition.

Nonbiogenic structures

Lithologic, textural, or color discontinuities are identified as boundaries. Boundaries between different lithologies are classified as sharp, gradational, or wavy. If alternation of different lithologies occurs at a <30 cm scale, the lithologies are described together as a bedded or laminated unit. Bedding and lamination, respectively, are defined as follows:

• Thick bedded = >30 cm.

• Medium bedded = >10 cm and ≤30 cm.

• Thin bedded = >3 cm and ≤10 cm.

• Very thin bedded = >1 cm and ≤3 cm.

• Thickly laminated = >0.3 cm and ≤1 cm.

• Thinly laminated = ≤0.3 cm.

Bedding/lamination features include

• Fining-upward bedding: a layer with grains displaying a gradual uphole decrease in size;

• Coarsening-upward bedding: a layer with grains displaying a gradual uphole increase in size;

• Tilted bedding: layers of sediment that are inclined relative to the core barrel but have no clear relationship to adjacent layers;

• Cross bedding/cross lamination: layers of sediment that are inclined and truncated relative to the base and top of the set in which the inclined layers are grouped;

• Parallel laminae: alternation of parallel laminae of differing composition and/or color; and

• Wavy/undulated laminae: parallel laminae with gentle sinuosity or slightly diffuse contacts.

Other nonbiogenic structures include

• Bottom cast: preservation of seafloor irregularity caused by seafloor scouring due to a high flow regime or density contrasts between adjacent lithologies;

• Soft-sediment deformation: wavy to disorganized, twisted, or folded layers interbedded with nondeformed strata; and

• Vein: a mineral-filled fracture.

Structures associated with coring disturbances include

• Biscuits: the core is partitioned into biscuits and highly sheared zones by rotation of the core in the core barrel during extended core barrel (XCB) coring;

• Pieces: the core is broken into pieces, usually from extrusion from the core barrel due to gas expansion or extraction from the core catcher;

• Flow-in: the core is intruded by other materials such as sand due to sucking during retrieval of the APC;

• Fall-in: material falls from the hole walls onto subsequent cores and is found at the top of Section 1 and smeared along the core liner;

• Soupy: the top of the core is very loose and watery from exposure to seawater, exposure to seawater and sediment from the walls of the hole being scraped into the core barrel from shooting short cores with the APC, or from washing during XCB coring;

• Puncture: a disturbance that results from piercing a small hole through the core liner for degassing that causes a loss of sediment around the holes;

• Gas expansion: part of the core is partitioned into pieces and voids due to the expansion of interstitial gas;

• Crack: an open fracture caused by gas expansion, desiccation, or disturbance of the sediment after drilling; and

• Void: a large fracture (>2 cm) caused by gas expansion or disturbance of the sediment after drilling.

The intensity of coring disturbance is described as follows:

• Slightly disturbed: the original structure of the core is still visible;

• Moderately disturbed: the original structure of the core is fairly visible; or

• Highly/severely disturbed: the original structure of the core is lost.

Biogenic structures

Biogenic structures are structures produced by macrofauna that lived on or in the sediment during deposition (trace fossils). The intensity of bioturbation is first described according to the following classes:

• Absent or nonvisible (0%),

• Slight (0%–30%),

• Moderate (>30%–60%), and

• Strong (>60%–100%).

These percentages were estimated relative to the surface area of the split core and were conducted on the portions of sediment affected by obvious burrowing, that is, where the color/texture of the dominant lithology is disrupted by features (tubular, mottled, etc.) having colors or textures imported from adjacent layers of different composition. Note that it may be impossible to distinguish between sediment of homogeneous composition without any bioturbation and sediment that has been effectively homogenized by bioturbation. The shape and geometry of bioturbation structures (e.g., vertical, horizontal, and mottled) are described and, when possible, attributed to one of four major ichnofacies types (Planolites, Zoophycos, Chondrites, and Skolithos).

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