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doi:10.2204/iodp.proc.336.102.2012

Lithostratigraphy

Sediment cores recovered during Expedition 336 were split into archive and working halves. The cores were oriented (top and bottom) and referenced, and the archive halves underwent several stages of analyses and measurements, including density, susceptibility, and velocity measurements and natural gamma radiation (NGR) measurements. The archive halves were also imaged, run through the Section Half Multisensor Logger (SHMSL), and measured for physical properties (wet mass and dry mass), velocity, and thermal conductivity. The archive halves were then described.

Lithostratigraphy was determined with the help of some of the analytical and descriptive techniques listed above; for sedimentologic observation, visual (macroscopic) core description and thin section and smear slide description were also used. These procedures are outlined below.

Visual (macroscopic) core description

Descriptive data were entered using the DESClogik program (see DESClogik user guide in Technical Documentation [iodp.tamu.edu/​tasapps/]) and uploaded to the LIMS database. Spreadsheet templates were customized for Expedition 336 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 allowed quantification of 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 uploaded in the IODP-USIO Sample Master program. The descriptive data were used to produce visual core description (VCD) graphic reports. Symbols used in sedimentary VCDs are shown in Figure F2.

Sediment colors were determined qualitatively using the Munsell soil color charts (Munsell Color Company, Inc., 2000).

Sediment components and percentages in the core were determined using a hand lens, binocular microscope, smear slide examination, or thin section.

Core imaging

Core section imaging included both whole- and half-core scanning on the Section Half Imaging Logger (SHIL) (see SHIL quick start guide in Technical Documentation [iodp.tamu.edu/​tasapps/]). The SHIL incorporates a line-scan camera that 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. The line-scan camera imaged 10 lines/mm to create a high-resolution TIFF file (see “Raw TIF image” in the LIMS database). 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 because of differences in section-half surface height. High- and low-resolution JPG files were subsequently created from the high-resolution TIFF file (an uncropped and lightened JPG [see “Uncropped image” in the LIMS database], and a cropped and lightened JPG [see “Cropped image” in the LIMS database]). The system generates a ruler according to curated lengths of the cropped version.

Whole-core sections were wetted before being scanned to improve image quality. Half-core sections (flat face of split cores) were imaged as soon as possible after splitting to minimize color changes that occur through oxidation. These half-core sections were scanned dry to preserve the quality and accuracy of the pictures. Before scanning soft-sediment half-core sections, the split surfaces of the archive halves were scraped lightly with a glass slide to create an even surface. All images were uploaded to the LIMS database.

Smear slides

Smear slides were prepared using standard preparation techniques, as outlined in Myrbo (2007) and Bown (1998). Samples were treated with kerosene to disaggregate the grains, as outlined in Riedel and Sanfilippo (1977).

A sample was broken into small pieces, dried thoroughly in an oven or on a hot plate at ~80°–100°C, and then covered with kerosene while still warm. When the sediment was saturated (usually within a few minutes), the excess liquid was decanted and the sample was immediately covered with water. The sample was then allowed to stand with occasional stirring until it was judged that no further disaggregation would occur, ~5–30 min.

Thin sections

Thin sections were created on board when representative lithified sediments were encountered. Thin sections generally provide less biased samples of whole rock than do smear slides, and they allow for more accurate identification of the minerals present.

Sediment and hard rock classification

Sediment names indicate the degree of sediment induration (e.g., sand versus sandstone, silt versus siltstone, mud versus mudstone, ooze versus chalk, or limestone). One or two modifiers precede the principal name.

Sediment lithology

Sediments that contain more than ~70% calcareous components, the majority of which are secreted by pelagic organisms (planktonic foraminifers and calcareous nannofossils), are called ooze if they are soft, chalk if they are firm, and limestone if they are hard. The term “ooze” is used to describe <2 mm calcareous unlithified sediments containing >90% carbonate.

When clasts of hard rock were encountered within unlithified lithologies, grain size was described using the classification of Wentworth (1922). Sand clast sizes include very fine sand (63–125 µm), fine sand (125–250 µm), medium sand (250–500 µm), coarse sand (500 µm to 1 mm), and very coarse sand (1–2 mm). Granules are between 2 and 4 mm, pebbles are between 4 and 64 mm, and cobbles are between 64 and 256 mm. The degree of rounding of the clasts or grains and the degree of sorting were also determined. In unlithified lithologies, when sediment contains hard rock clasts of pebble size (4–64 mm) in a matrix of ooze, it is named “muddy gravel.”

Indurated lithologies

The classification of Dunham (1962) was used for limestones, typically when thin sections were described. When mud (micrite) is absent and the texture is grain supported, the limestone is called “grainstone”; when carbonate mud is present and the texture is grain supported, the limestone is named “packstone”; when carbonate mud is present and the texture is mud supported, with at least 10% grains, the limestone is “wackestone,” and with <10% grains the limestone is named “mudstone.”

Modifiers and suffixes

Modifiers added to the principal term “ooze” consist of the names of the major fossil types and may include a suffix such as “-bearing” or “-rich.” The fossil type that is least abundant is named first. For example, “foraminifer-bearing nannofossil-rich ooze” describes an ooze made of a majority of nannofossil tests with foraminifer tests; “nannofossil-rich ooze” means that nannofossils represent the dominant part of the ooze. Biogenic components are not described in textural terms.

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