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doi:10.2204/iodp.proc.340.102.2013

Lithostratigraphy

This section outlines the procedures used to document the composition, texture, alteration, and sedimentary structure of geologic materials recovered during Expedition 340. These procedures include visual core description, graphical core logging, smear slide and petrographic thin section description, digital color imaging, and color spectrophotometry. Because many of the geologic techniques and observations used to analyze sedimentary material are similar to those used to analyze igneous material, the methods for both are presented together. All acquired data were uploaded into the IODP-USIO Laboratory Information Management System (LIMS), and observations were entered using the DESClogik application in Tabular Data Capture mode. For consistency, our procedures and database templates closely followed the methods for core descriptions from recent IODP volcanic and volcaniclastic expeditions, in particular IODP Expedition 330, Louisville Seamount Trail (Expedition 330 Scientists, 2012), and IODP Expedition 334, Costa Rica Seismogenesis Project (Expedition 334 Scientists, 2012).

Core sections available for sedimentary, petrographic, and structural observation and interpretation included both working and archive halves. Sections dominated by soft sediment were split using a thin wire held in high tension. Pieces of hard rock were split with a diamond-impregnated saw so that important compositional and structural features were preserved in both the archive and working halves. The split surface of the archive half was then assessed for quality (e.g., smearing or surface unevenness) and, if necessary, scraped lightly with a glass slide or spatula. After splitting, the archive half was imaged with the SHIL and then analyzed for color reflectance and magnetic susceptibility using the SHMSL.

Following imaging, the first step in describing the recovered core was identifying unit boundaries on the basis of lithologic changes, color, grain size, the presence of volcaniclastic sediment or sedimentary intercalations, structure, and alteration. Archive sections of sediment cores were macroscopically described for lithologic, sedimentary, volcanic, and structural features. Lithostratigraphic units were characterized by visual inspection; smear slide samples were used to determine sedimentary constituents and abundances to aid in lithologic classification.

Based on preliminary visual descriptions and physical properties data, thin section samples and samples for XRD and inductively coupled plasma–atomic emission spectroscopy (ICP-AES) were extracted from working half sections. All descriptions and sample locations were recorded using curated depths and then documented on visual core description (VCD) graphic reports. A graphical core log of the core was also hand drawn, scanned, and added to the VCD (see “Graphical core logs”).

DESClogik application

Descriptive data were input into the LIMS database using the DESClogik tabular entry application.

The DESClogik Expedition 340 Core Description template consists of a spreadsheet with a series of tabs. Key features expected to occur in the cores were grouped together in a General tab. These features represented observations that were made for every interval cored. More detailed information could be added into other tabs as needed for specific intervals.

The General tab included all information necessary to generate a VCD: drilling disturbance intensity, major lithology (and abundance and maximum grain size), minor lithology (and abundance and maximum grain size), clast percentage, percentage of seven clast types (see “Clast textural types”), clast angularity, matrix alteration intensity, deformation structures, and structural strike and dip.

The remaining template tabs were used to record clast composition; macroscopic structures; tephra layers; veins, vesicles, and halos; smear slides; alteration; thin section mineralogy, alteration, and structure; and so on. A brief synthesis of the section was recorded in the Section summary tab, and a summary of the entire core was recorded in the Core summary tab.

Drilling disturbance

Drilling disturbance indicates the intensity of sediment disturbance caused by the drilling process. The degree of disturbance within soft sediment is characterized using the nomenclature of Ocean Drilling Program (ODP) Leg 180 (Taylor, Huchon, Klaus, et al., 2000):

• Slightly disturbed: bedding contacts are slightly deformed.
• Moderately disturbed: bedding contacts have undergone extreme bowing.
• Extremely disturbed: bedding is completely deformed as flow-in and other soft stretching and/or compressional shearing structures attributed to coring/drilling.

Major and minor lithologies

Major and minor lithologies were chosen from standard ODP/IODP lithology terminology (Fig. F2). In some cases the lithology (e.g., basaltic) needed to be divided by texture (e.g., pumiceous). The DESClogik value list initially contained every lithology ever encountered during IODP drilling, but this extensive list was simplified to those lithologies that were likely to be encountered during this expedition. This simplification was helpful when we described large numbers of core sections in short periods of time.

Clast percentage

Clast percentage refers to the percentage area covered by clasts within the described interval.

Clast textural types

Seven types of clasts were identified in the core: pumice, scoria, lava-massive, lava-vesicular, volcaniclastic (reworked), sedimentary, and other (Fig. F3).

1. Pumice clasts have white to yellow color. These clasts are generally poorly angular to rounded, as they result from pumice fallout and pumice pyroclastic flows. They are highly vesiculated (≤80%). Vesicles are generally spherical to subspherical. Glass is poorly microcrystallized.
2. Scoria clasts have a dark color and an irregular shape and are vesiculated to poorly vesiculated with large and irregular vesicles. The largest vesicles result from the coalescence of smaller ones. The groundmass is always microcrystalline.
3. Massive lava clasts are dense and without vesicles. They are gray to dark in color depending on composition (andesite to basaltic andesite) (LeBas et al., 1992) and alteration. In some cases when they are oxidized they take on a red color. They are generally phenocryst rich (35%–55%). They are part of massive lava domes and, in only a few cases, of lava flows.
4. Vesiculated lava clasts have the same characteristics as massive lava clasts but contain more or less abundant vesicles. They result mainly from the outer parts of lava domes or from superficial parts of lava flows. They can have a scoriaceous aspect.
5. Volcaniclastic clasts are sedimentary facies composed of volcanic (mainly pyroclastic) material. This material may include ash, lapilli, blocks, bombs, and tuffs.
6. Sedimentary clasts are composed of limestone. Limestones are white carbonate rocks from the coral platform.
7. The category “other clasts” contains all the clasts not described before: cumulates originating from magma chambers, hydrothermalized lavas from hydrothermal systems, basic enclaves, and mud clasts. In Expedition 340 cores, mud clasts and hydrothermalized lavas from hydrothermal systems are the dominant clasts observed in this category.

Grain size

Grain size refers to the coarse tail (maximum size) of the matrix. Sizes of outsized clasts are described in a separate data entry. The following grain sizes were used in DESClogik for the matrix:

• Very fine mud,
• Fine mud (where silt grains were not visible),
• Silt-fine-mud (grains visible but human eye cannot quantify size),
• Very fine sand (<187 µm),
• Fine sand (<250 µm),
• Fine medium sand (<375 µm),
• Medium sand (<500 µm),
• Coarse sand (<1 mm),
• Very coarse sand (1–2 mm),
• Granules (2–4 mm), and
• Pebbles (4–64 mm).

Sedimentary structures

Sedimentary structures include those formed by the migration of bedforms or by action of organisms (bioturbation) and may include features such as ripple cross-lamination or planar lamination. When possible, the strike and dip of such features were noted. Figure F3 contains the full set of sedimentary structures seen within the cores.

Alteration intensity

Alteration characteristics were described from visual observations of the core section archive halves (Fig. F3). The core description team adopted a basic methodology that involved identifying what was altered (e.g., debris avalanche block, avalanche matrix, or lava clasts) the degree of alteration, and alteration color. Color identification was achieved using simplified names based on Munsell Soil Color Charts (Munsell Color Company, Inc., 1994). Notes on the form of alteration (e.g., veins or halos) and the alteration mineral assemblages were recorded. However, detailed descriptions and identifications were not deemed necessary for achieving Expedition 340 scientific objectives and were therefore not completed on board the ship. These descriptions can be determined on shore at a later date if necessary.

Degree of alteration is represented graphically on the VCDs as alteration intensity and is defined according to the following ranges:

• Fresh = <2 vol%.
• Slight–moderate = 2–10 vol%.
• Moderate–high = >10–50 vol%.
• High–complete = >50–100 vol%.

Graphical core logs

Describing sequences of clastic rocks in ancient rocks or modern cores is most commonly done through drawings called graphical core logs (Bouma, 1962). These logs comprise a horizontal axis of grain size and a vertical axis of core depth. Annotated comments can then be added to the pictorial description. A full key to the symbols used during Expedition 340 is provided in Figure F2.

Graphical core logs were used during Expedition 340 because it is much quicker to draw the sequence of deposits than write a text description (and then enter that text via tabs or drop-down menus, as in DESClogik). The second reason for using graphical core logs is that it is easier for a reader to understand a picture than dense text. Graphical core logs have been widely employed over many decades (e.g., Bouma, 1962). DESClogik imposes predefined categories on users and necessitates placement of sharp boundaries between unit descriptions. Visual core logging, as completed during this expedition, allows a drawing of the core with minimal interpretation. It also allows more (necessary) flexibility in allowing the key to evolve and grow as observations of the cores are made. DESClogik requires a format and key to be defined before the cores are seen.

Grain size

Matrix grain size was estimated using a comparator card, which allows the describer to compare the core grain size to reference sizes on the card. The grain size categories used were mud (silt grains were not visible), muddy silt, <125 µm, <187 µm, <250 µm, <375 µm, <500 µm, <1 mm, <2 mm, <1 cm, and <10 cm. The use of grain size comparators ensures that the grain size estimated is the coarser tail of the grain size distribution (e.g., about the 95th percentile of the full distribution; Talling et al., 2004). The maximum and average size of clasts was noted in a separate column.

Boundaries and grading patterns

On the graphical core logs, sharp boundaries are denoted by a half arrow, and gradational boundaries are shown by triangles whose rotation shows whether the interval is inversely or normally graded. Erosional boundaries are shown by an inclined wavy line (Fig. F2).

Annotated comments

Where appropriate, written annotations were added to the graphical core log to clarify key observations (e.g., clast types, sizes, and percentage abundance). These comments are separated from the drawn log, as they may also on occasion involve interpretation rather than observation (Fig. F2). Similarly, the left-hand column is used to define layers that are definitely turbidites (e.g., as shown by cross-bedding or other features), tephra (ash fall or distal turbidites), blocky volcanic debris avalanches, or deformed intervals of more bedded seafloor sediment. This latter division of landslide deposits follows that of Watt et al. (2012).

Microscopic visual core description

Thin sections

Thin section analyses of sampled hard rock clasts were used to complement and refine macroscopic core observations, and this information was added to the database through DESClogik. Samples that represented a new lithologic or textural type were selected for thin section preparation. Phenocryst assemblages (and their modal percentages and sizes), groundmass, and alteration phases were determined, and textural descriptions were constructed. Textures were defined at the microscopic scale according to the degree of crystallinity of the groundmass (holohyaline to holocrystalline). A visual estimate of the modal percent of the phenocrysts was made, average crystal size for each mineral phase was determined, and the mineral shape and mineral habit were also determined. The following textural terms were used to describe the habit of crystals: “euhedral,” “subhedral,” “anhedral,” and “interstitial.” Grain shape was divided into four classes:

1. Equant (aspect ratio = <1:2),
2. Subequant (aspect ratio = 1:2 to 1:3),
3. Tabular (aspect ratio = 1:3 to 1:5), and
4. Elongate (aspect ratio = >1:5).

All observations were entered into the LIMS database using the Expedition 340 DESClogik thin section template.

Smear slides

Smear slide information was also added to the database using DESClogik. Smear slides are useful for identifying and reporting basic sediment attributes (texture and composition) in soft sediment, but results are not quantitative. Similar to the procedure used during Expedition 330, we estimated the abundances of biogenic and volcaniclastic constituents with the help of a visual comparison chart (Wentworth, 1922). Descriptive results are tabulated as visual percentage estimates in the LIMS database, with values grouped into the following broad range of categories:

• D = dominant (>50%).
• A = abundant (>20%–50%).
• C = common (>5%–20%).
• P = present (>1%–5%).
• R = rare (0.1%–1%).
• T = trace (<0.1%).

Visual core descriptions

VCDs were generated from descriptive information retrieved from LIMS. VCDs combine key information input through DESClogik along with physical properties measurements (e.g., Fig. F4):

• Depth in meters below seafloor (mbsf),
• Scale for core section length,
• Sample piece number,
• Symbolized drilling disturbance intensity,
• Scanned digital image of the archive half,
• Lithologic unit number,
• Line chart displaying drilling disturbance by degree (slight, moderate, extreme, or none),
• Stacked line chart displaying clast composition and percentage (pumice, scoria, massive lava, vesicular lava, reworked [volcaniclastic], limestone [sedimentary], or other),
• Line chart displaying total clast content (1%–100%),
• Symbolized structural information,
• Structural measurements of dip direction and dip angle,
• Line chart displaying matrix alteration intensity (fresh [<2%], slight to moderate [2%–10%], moderate to high [10%–50%], or high to complete [50%–100%]),
• Sample type and position of intervals selected for different types of shipboard analytical studies,
• Neutron density, and
• Magnetic susceptibility.

Graphical core logs (see “Graphical core logs”) drawn on board the research vessel were scanned and added as PDFs in a final column within the VCDs (Fig. F4). This strategy was adopted to combine the advantages of the graphical core logging approach and the data entry system via DESClogik.

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